Process for forming fibrous structures with predetermined Z-fiber distributions

ABSTRACT

The invention relates to the field of forming fibrous structures by needling processes. A process is provided for achieving a desired Z-fiber distribution in a fibrous structure by controlling transport depth during formation of the fibrous structure in a series of needling passes. Each needling pass generates a portion of Z-fiber bundles that penetrate adjacent fibrous layers, the Z-fiber bundles penetrating only those adjacent fibrous layers that need more Z-fiber bundles to attain a predetermined number of Z-fiber bundles within each fibrous layer.

This application is a continuation-in-part of prior application Ser. No.08/279,608 filed Jul. 25, 1994, now U.S. Pat. No. 5,515,585.

BACKGROUND OF THE INVENTION

The invention relates to the field of forming fibrous structures fromfibrous materials. More particularly, the invention relates to a processfor cohering a multitude of fibrous layers with a predetermineddistribution of Z-fiber bundles that pass between adjacent fibrouslayers.

Needling processes for forming fibrous preform structures for use incomposite structures have been known for many years. U.S. Pat. No.3,772,115 to Carlson et al. describes a process whereby several fibrouslayers may be needled together simultaneously or in a series of needlingsteps. The Carlson et al. needling process involves repeatedly driving amultitude of barbed needles into the fibrous layers. The barbed needlesdisplace fiber within the layers which causes the layers to adhere intoa coherent structure. The structure may be incrementally formed byadding layers in a series of needling steps if the final structure istoo thick to allow the needles to pass all the way through. The fibrouslayers comprise carbon or graphite fabric, or precursors thereof. Afibrous preform structure formed according to the process may be furtherprocessed into a carbon/carbon composite structure by deposition of acarbon matrix within the fibrous preform structure that binds the fiberstogether. The Carlson et al. process may be used to form variouscomposite structures, including carbon/carbon brake disks.

A similar process is disclosed in Great Britain Patent Specification1,549,687, published Aug. 1, 1979. This also discloses a process forforming a carbon/carbon composite material. The fibrous layers may becomprised of oxidized polyacrylonitrile cloth which are needled togetherin a series of needling steps. In one example, the process was used toform a carbon brake disk.

A more recent process is disclosed by U.S. Pat. No. 4,790,052 to PierreOlry. The goal is to produce a fibrous preform structure having a highdegree of uniformity. This purportedly is accomplished by needlingsuperposed layers together with a "uniform density" of needlingthroughout the thickness of the article. The initial depth ofpenetration is determined as a function of the number of layers to betraversed by the needles, for example about twenty layers. The Olry etal. process attempts to keep this depth constant throughout formation ofthe fibrous preform structure by lowering the fibrous structure awayfrom the needles a distance equal to the thickness of a needled layereach time a new layer is added.

U.S. Pat. No. 4,955,123, issued Sep. 11, 1990 and PCT Publication WO92/04492, published Mar. 19, 1992, both to Lawton et al., describe aprocess whereby a brake disk is formed by needling together sectors ofan annulus. The fibrous structure is lowered the thickness of a needledlayer each time a new layer is added. U.S. Pat. No. 5,338,320, issued asa continuation in part from the '123 patent on Feb. 14, 1995, to Smithet al. describes a process wherein outer layers of a preform are"enriched" with staple fiber in a needling process. Enriching the outerlayers in the Smith et al. process apparently increases mechanicalstrength in the outer layers and improves wear characteristics of aresulting disk brake.

In the fibrous preform art, the displaced fibers generated by theneedling process are referred to as "Z-fibers" since they are generallyperpendicular to the layers comprising a fibrous preform structure. TheZ-fiber distribution throughout a brake disk can have a profound effecton disk wear life and on performance of the brake disk in slowing orstopping an aircraft. Therefore, a fibrous structure having apredetermined Z-fiber distribution and a method for forming a fibrousstructure with a predetermined Z-fiber distribution are desired.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a process is provided forforming a fibrous structure, comprising the steps of:

superposing and needling a multitude of fibrous layers together in aseries of needling passes that generate a multitude of Z-fiber bundleswithin the multitude of fibrous layers, each needling pass generating aportion of the multitude of Z-fiber bundles that pass through adjacentfibrous layers, each portion of the multitude of Z-fiber bundlespenetrating only those adjacent fibrous layers that need more Z-fiberbundles to attain a predetermined number of Z-fiber bundles within eachfibrous layer in the multitude of fibrous layers.

According to another aspect of the invention, a process is provided forforming a fibrous structure, comprising the steps of:

cohering a multitude of fibrous layers in a series of needling passes,each needling pass comprising the steps of superposing at least onefibrous layer over fibrous layers previously cohered by previousneedling passes, and generating Z-fiber bundles that extend from the atleast one fibrous layer into the previously cohered fibrous layers, eachof the previously cohered fibrous layers receiving Z-fiber bundlesduring subsequent needling passes until the number of Z-fiber bundles inthat fibrous layer reach a predetermined number of Z-fiber bundles forthat layer, each fibrous layer in the multitude of fibrous layersthereby having a predetermined number of Z-fiber bundles for thatfibrous layer.

According to another aspect of the invention, a process is provided forforming a fibrous structure, comprising the steps of:

determining a desired number of adjacent fibrous layers to be penetratedby Z-fiber bundles for each needling pass such that the sum of Z-fiberbundles generated in each layer by each needling pass equals apredetermined number of Z-fiber bundles for that fibrous layer; and,

superposing and needling the multitude of fibrous layers together in aseries of needling passes that generate the multitude of Z-fiber bundleswithin the multitude of fibrous layers, the Z-fiber bundles generated byeach needling pass penetrating the desired number of adjacent fibrouslayers for that needling pass, thereby achieving the desired number ofZ-fiber bundles for each fibrous layer in the multitude of fibrouslayers.

According to another aspect of the invention, a process for making afibrous structure is provided, comprising the steps of:

forming a multitude of Z-fiber bundles passing between fibrous layerswithin a multitude of superposed fibrous layers, the multitude ofsuperposed fibrous layers including a first group having a lower fibrouslayer and an upper fibrous layer, each fibrous layer within the firstgroup having a portion of the multitude of Z-fiber bundles originatingin that fibrous layer and passing through a number of fibrous layersdisposed beneath that fibrous layer without passing through all of thefibrous layers disposed beneath that fibrous layer, the number offibrous layers increasing from the lower fibrous layer to the upperfibrous layer.

According to another aspect of the invention, a fibrous structure isprovided, comprising:

a multitude of superposed fibrous layers including a first group havinga lower fibrous layer and an upper fibrous layer; and,

a multitude of Z-fiber bundles passing between fibrous layers within themultitude of fibrous layers, each fibrous layer within the first grouphaving a portion of the multitude of Z-fiber bundles originating in thatfibrous layer and penetrating subjacent fibrous layers without passingthrough all of the subjacent fibrous layers, the portions of Z-fiberbundles penetrating an increasing number of fibrous layers from thelower fibrous layer to the upper fibrous layer.

Thus, the invention provides a means for controlling Z-fiberdistribution within a fibrous structure, and includes a fibrousstructure having a desired Z-fiber distribution. The invention isparticularly useful for making fibrous preform structures forcarbon/carbon aircraft brake disks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a general schematic arrangement of a needling apparatussuitable for use with the invention.

FIG. 2 depicts a detailed view of a needling process using the FIG. 1apparatus.

FIG. 3 depicts a detailed view of a Z-fiber bundle generated during theFIG. 2 needling process.

FIG. 4A depicts a first part of a method for determining a minimum fibertransport distance.

FIG. 4B depicts a second part of a method for determining a minimumfiber transport distance.

FIG. 5A depicts a first needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 5B depicts a second needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 5C depicts a third needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 5D depicts a fourth needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 6 depicts estimated surface position versus needling pass for theFIGS. 5A-5D needling process.

FIG. 7A depicts a first needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 7B depicts a second needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 7C depicts a third needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 7D depicts a fourth needling pass in a series of needling passesaccording to an aspect of the invention.

FIG. 8 depicts estimated surface position versus needling pass for theFIGS. 7A-7D needling process.

FIG. 9 depicts a compaction curve showing post-needled average layerthickness of the layers comprising a fibrous preform structure versusnumber of layers according to an aspect of the invention.

FIG. 10 depicts compaction curves showing layer thickness of an airlaidweb versus number of needling passes according to an aspect of theinvention.

FIG. 11 depicts a schematic of a diagonal matrix for quantifyingpermanent fiber transport and determining a cumulative quantity ofpermanent transport for each layer, and a number of Z-fiber bundles perunit area according to an aspect of the invention.

FIG. 11A depicts a detailed view of the area indicated as 11A in FIG.11.

FIG. 11B depicts a detailed view of the area indicated as 11B in FIG.11.

FIG. 11C depicts a detailed view of the area indicated as 11C in FIG.11.

FIG. 12 depicts the relationship of certain variables to needlingprocess geometry.

FIG. 13 depicts a schematic of a diagonal matrix for quantifyingpermanent fiber transport and for manipulating a cumulative quantity ofpermanent transport for each layer, and a number of Z-fiber bundles perunit area according to an aspect of the invention.

FIG. 13A depicts a detailed view of the area indicated as 13A in FIG.13.

FIG. 13B depicts a detailed view of the area indicated as 13B in FIG.13.

FIG. 14A depicts a table useful for determining a desired fibertransport depth for needling pass 2 of FIG. 13.

FIG. 14B depicts a table useful for determining a desired fibertransport depth for needling pass 3 of FIG. 13.

FIG. 14C depicts a table useful for determining a desired fibertransport depth for needling pass 4 of FIG. 13.

FIG. 15 depicts a sectional view of a fibrous structure having apredetermined Z-fiber distribution according to an aspect of theinvention.

FIG. 16 depicts a Z-fiber bundle typically generated by a needlingprocess.

FIG. 17 depicts a sectional view of an annular brake disk formed from afibrous structure similar to FIG. 15, according to an aspect of theinvention.

FIG. 18 depicts a sectional view of a fibrous structure having apredetermined Z-fiber distribution according to an aspect of theinvention.

FIG. 19 depicts a sectional view of a fibrous structure having apredetermined Z-fiber distribution according to an aspect of theinvention.

DETAILED DESCRIPTION

The invention relates to a process for forming fibrous preformstructures. Fibrous preform structures according to the invention aresuitable for subsequent processing in which a binding matrix isdeposited within the preform structure thus forming a compositestructure. The invention is especially suited for forming fibrouspreform structures suitable for subsequent processing into carbon/carbonstructures such as aircraft brake disks. Subsequent processingconventionally involves pyrolyzing the preform structure (if it isformed from a precursor material), and depositing a binding carbonmatrix. Depositing the carbon matrix within the preform structure may beaccomplished according to known techniques such as carbon vaporinfiltration and carbon vapor deposition (CVI/CVD), or by repeatedlyimpregnating the substrate with a carbon bearing pitch or resin which issubsequently charred, or by any equivalent process. The invention is notdirected to forming the binding matrix or densification of the fibrouspreform structure, as these techniques are known in the art. Thoughdescribed in relation to carbon/carbon composites, it is clearlycontemplated that the invention may be used with equal utility forforming fibrous preform structures suitable for subsequent processinginto ceramic composite structures and carbon/ceramic compositestructures.

Various aspects of the invention are described as follows in relation toFIGS. 1 through 13 wherein like numbered components are equivalent.Referring specifically to FIG. 1, a needling apparatus 8 isschematically depicted. Apparatus 8 is suitable for forming a fibrouspreform structure by cohering several fibrous layers together beginningwith at least two fibrous layers to which additional fibrous layers areadded in a series of needling passes, and is presented in FIG. 1 as onlya general arrangement since such devices are well known in the art.Examples of apparatuses suitable for use with the invention aredescribed in U.S. Pat. No. 4,790,052 to Olry (hereinafter the '052patent); U.S. Pat. No. 4,955,123 to Lawton et al. (hereinafter the '123patent); and, U.S. Pat. No. 5,217,770 to Morris, Jr. et al. (hereinafterthe '770 patent). FIG. 1 is intended to present only the needling zoneof any such apparatus. Thus the invention is adaptable for use with anapparatus for forming a sheet or "board" as described in the '052patent, or for forming an annulus as described in the '123 and '770patents. Any variation in shape of the fibrous preform structureproduced is considered to be within the purview of the invention.

Still referring to FIG. 1, a fibrous preform structure 20 is shown inthe process of being formed in apparatus 8. The fibrous structure 20 isdisposed on a support or bedplate 12 beneath a multitude of feltingneedles 14 mounted in a needle board 16. Support 12 is penetrable by theneedles 14, and may be formed from a penetrable material such as foamedplastic or brush bristles, or an impenetrable material such as metal orplastic with holes aligned with the needles 14 to allow penetration.Fibrous structure 20 is comprised of fibrous layers with a top layerdefining an exposed surface 44. The fibrous structure is then subjectedto a needling pass in which the multitude of felting needles 14 arerepeatedly driven into the fibrous structure 20 through exposed surface44 as the fibrous structure 20 is passed beneath the felting needles inthe direction of arrow 34. As used herein, the term "fibrous structure"refers to all the fibrous layers disposed on the support 12 beneath thefelting needles 14 during a given needling pass. Layers may be added tothe fibrous structure at one or more needling passes, but it is notnecessary to add a fibrous layer to the fibrous structure at everyneedling pass.

The felting needles 14 are arranged in an array as is known in the art.Several rows may be arranged parallel to each other so the entire widthof the fibrous structure 20 may be subjected to needling with eachneedling pass. The array of felting needles 14 defines a needling zone32.

The felting needles 14 are driven by means of a drive mechanism 18 thatdrives needle board 16 through a fixed range of travel in areciprocating motion. The multitude of felting needles thereby displacefibers among layers of the fibrous structure 20 producing "Z-fiber"bundles that pass between layers generally perpendicular to the layerinterfaces. Additional layers are disposed over previous layers andsubjected to additional needling passes which adheres the additionallayers to the previous layers. Additional layers are added until a finaldesired thickness is established. The fibrous structure 20 may then besubjected to further needling passes without adding additional layers.The finished fibrous preform structure 20 can be processed in subsequentoperations, in known manner, as previously described.

A support adjustment mechanism 22 adjusts the support position relativeto the multitude of felting needles 14 in a manner well known in theart. Here, the support adjustment mechanism comprises jackscrew 24 andmotor/gearbox 26. A controller 28 controls the support adjustmentmechanism 22 via control line 30 as necessary in order to preciselyposition the support 12 relative to the multitude of felting needles 14.Support 12 is driven in the direction of arrow 34 such that themultitude of felting needles may be repeatedly driven into the exposedsurface 44 along the length of fibrous structure 20. The support may besubsequently driven in the direction of arrow 36 thereby needling thefibrous preform structure in the opposite direction, as described in the'052 patent. The support would normally be driven in only one directionif an annular shape is being formed as described in the '123 or '770patents. Also, the support 12 may be continuously driven as the needlesare driven into fibrous structure 20, or the support may be synchronizedwith drive mechanism 18 such that the support 12 momentarily stops whenthe needles 14 are driven into fibrous structure 20. Any such variationsare considered to be within the purview of the invention. Also, thevarious components of apparatus 8 may be oriented in various obviousways without departing from the invention. For example, apparatus 8could be rotated onto a side or even inverted if a particularapplication necessitated such an arrangement.

A process according to an aspect of the invention is depicted in FIG. 2,wherein a fibrous structure 20 comprising at least two fibrous layers40a-40i is disposed beneath the multitude of felting needles 14 onsupport 12. As depicted, the fibrous structure 20 may comprise amultitude of superposed layers, and the stack of layers may become sothick that the felting needles 14 do not penetrate all the way throughthe fibrous structure. A top fibrous layer 40a is disposed over loweradjacent layers 40b, 40c, 40d, 40e, 40f, 40g, 40h, and 40i with the toplayer 40a defining an exposed surface 44. In this example, top layer 40ais not adhered to layer 40b until it is subjected to a needling pass inwhich fibrous layers 40a-40i are passed beneath the multitude of feltingneedles 14 while the multitude of felting needles 14 are repeatedlydriven through the exposed surface 44 into the fibrous structure, asshown in phantom, deep enough to permanently transport fiber from layer40a into lower adjacent layer 40b. The needling pass adheres top layer40a to layer 40b by permanently transporting fiber from the top layer40a into layer 40b and other lower adjacent layers.

According to an aspect of the invention, fiber is permanentlytransported from a chosen set 46 of layers for each needling pass. Theset of layers may change from one needling pass to the next. Choosingthe set of layers is a matter of preform design according to desiredfinal preform properties, and is not part of the invention. Inpracticing the invention, the set of layers includes at least the toplayer 40a. The set of layers preferably includes top layer 40a and atleast one adjacent layer 40b. In the example shown in FIG. 2, set 46includes layers 40a, 40b, and 40c. The set of layers could include morethan three layers in many applications.

Referring now to FIG. 3, a bundle 66 of Z-fibers generated by a singlefelting needle 14 is shown extending through the top layer 40a and downthrough the fibrous structure into layers 40b-40g. Felting needle 14 isshown to one side of the fibrous structure for the sake of clarity.Here, felting needle 14 has a tip 74 and comprises a first barb 68nearest the tip 74, a second barb 70 spaced from the first barb 68further away from tip 74, and a third barb 72 spaced from the secondbarb 70 further away from tip 74. Further sets of barbs may be spacedalong the felting needle 14 as shown. In the example shown, barbs 68,70, and 72 engage and transport fiber from layers 40a-40f when thefelting needle 14 is driven into those layers.

Fibers commonly used as carbon precursor materials such aspolyacrylonitrile fiber (PAN) and oxidized polyacrylonitrile fiber (OPF)have resilient qualities that cause the fibers to pull back to the layerfrom which they originate unless transported at least a minimumdistance. When the fibrous layers are made from long or continuousfilaments, permanent fiber transport may not be achieved unless thefibers are transported far enough to cause fiber rupture. Temperatureand humidity may have an effect. As used herein, the term "permanentfiber transport" means that the felting needles 14 transport fiber fromone layer into at least one adjacent layer during a given needling passthat stays transported after the felting needles 14 are withdrawn fromthe fibrous structure 20. Increasing fiber transport depth by as littleas 0.5 mm can result in a transition from no permanent fiber transportto permanent fiber transport. This was a surprising discovery. Thus, themultitude of felting needles 14 together engage and transport anaggregate of fiber from the set of layers during the needling processand less than 100% of the aggregate is permanently transported if thefiber comprising the fibrous layers tends to pull back to its layer oforigin as described.

Fiber length, fiber crimp, and fiber surface finish may also affectpermanent fiber transport. Staple fibers, or fibers that are crimped orhave a rough or scaled surface (similar to wool) may have less of atendency to pull back to their layer of origin. The minimum distance forfibrous layers comprised of fibers having one or more of thesecharacteristics may be much less than the minimum distance for fibrouslayers comprised of smooth, uncrimped, continuous fibers of the samecomposition. In this case, the minimum distance may have at least asmall threshold value since fiber must be transported at least somedistance in order to cohere the fibrous layers. With fibers having thesecharacteristics, essentially 100% of the aggregate of fiber engaged bythe multitude of felting needles 14 may be permanently transportedbecause the fiber comprising the fibrous layers has little tendency topull back to its layer of origin.

Thus, according to an aspect of the invention, a desired fiber transportdepth 48 is determined relative to the top layer 40a that is sufficientto achieve permanent fiber transport from the chosen set of layers 46without permanently transporting a significant amount of fiber from morethan the chosen set of layers 46. If the fibrous layers have resilientqualities, permanent fiber transport is not achieved until the transportdepth exceeds the minimum distance 50.

For example, fiber is transported to a depth 52 from layer 40a, which isgreater than minimum distance 50, which means that the fiber transportedfrom layer 40a is permanently transported. Likewise, fiber istransported to depths 54 and 56 from layers 40b and 40c respectively,which are greater than the minimum distance 50, which means that thefiber transported from layers 40b and 40c is permanently transported.Fiber is transported to depths 58, 60, 62, and 64 from layers 40d, 40e,40f, and 40g respectively, which are less than the minimum distance 50,which means the fiber transported from layers 40d through 40g is notpermanently transported. The fiber transported from those layers pullsback to those layers. Thus, fiber is permanently transported from theset 46 of layers without permanently transporting a significant amountof fiber from more than the set 46 of layers.

A preferred method for determining the minimum distance 50 is depictedin FIGS. 4A and 4B. Referring to FIG. 4A, a first fibrous layer 76 isdisposed over a second fibrous layer 78 with the first fibrous layer 76defining an exposed surface 80. Second fibrous layer 78 is disposed oversupport 12. The first fibrous layer 76 is substantially similar to toplayer 40a of FIG. 3, and the second fibrous layer 78 is substantiallysimilar to layer 40b of FIG. 3. Referring still to FIG. 4A, fiber istransported from the first layer 76 into the second layer 78 byrepeatedly driving a multitude of felting needles into the exposedsurface 80 through the first layer 76 and into the second layer 78 in amanner substantially similar to the needling process used to form thefibrous preform structure as depicted in FIGS. 1-3, using feltingneedles 14 substantially similar to those used in the needling process.Only one needle and only one barb are shown for the sake of clarity.Support 12 is perforated by holes 15 that are aligned with the needles14, which permit penetration of needle 14 into the support 12 as shown.The fibrous layers 76 and 78 are passed beneath the felting needles 14in the direction of arrow 90 as the needles are driven into the fibrouslayers, beginning with fiber transport depth 82. As shown, Z-fiberbundles 84 are created, but pull back to the top layer because fiber wasnot transported a great enough distance from top layer 76. The portionof the Z-fiber bundle that pulls back is shown in phantom. Referring nowto FIG. 4B, the fiber transport depth is increased (by moving support 12toward the multitude of felting needles 14) until the first layer justbegins to tack to the second layer 78, at which point the fibertransport depth 86 corresponds to the minimum distance 50. The firstlayer 76 begins to tack to the second layer 78 because the transportdepth 86 has a magnitude sufficient to permanently transport fiber fromthe top layer 76 thereby creating permanent fiber bundles 88 that bondthe two layers together.

Minimum distance 50 could also be determined by disposing fibrous layer76 over a multitude of previously needled layers (a fibrous structure or"board"), and performing the process of increasing needle penetrationdepth until the layer 76 is tacked down. This approach may moreaccurately quantify minimum distance 50 since the fibrous structurecomprises two layers (as shown in FIG. 4) at only the beginning of theprocess of forming a fibrous preform structure. With only two layers,the Z-fiber bundles extend below the bottom layer into the support.During most of the process, a top layer is disposed over a previouslycohered fibrous structure and is subjected to a needling pass (as shownin FIG. 2), and the Z-fiber bundles are completely enclosed within thefibrous structure (as shown in FIG. 3). However, quantifying the minimumdistance according to the FIG. 4 process has been found to be quitesuitable in the practice of the invention.

According to a preferred embodiment, each of fibrous layers 40 comprisethree unidirectional sub-layers of OPF fiber which are lightly needledtogether into a coherent layer, with the fiber direction of eachsub-layer rotated 60° relative to the adjacent sub-layer, as describedin EXAMPLE 1 of the '052 patent. The directional orientation of eachlayer is established by crosslapping a first unidirectional web onto asecond longitudinal web before the needling operation. The webs arepreferably formed from tows of OPF fiber, each tow being composed of320,000 filaments. OPF tow suitable for use in the practice of theinvention is available from RK Carbon Fibres Limited of Muir of Ord,Scotland, and Zoltek Corporation of St. Louis, Mo., U.S.A. UnoxidizedPAN fiber ("greige tow") is available from Courtaulds Advanced Materialsof Great Coats Grimsby, South Humberside, England. The minimum distanceestablished according to the FIG. 4 procedure with two of these OPFcrosslapped layers was about 6.5 to 7.0 millimeters with felting needlesaccording to catalogue number 15×18×36×3.5 C333 G 1002, available fromGroz-Beckert of Germany. The layers just began to tack at a penetrationdepth of about 6.5 mm and became fully tacked at about 7.0 mm. The OPFfibers of this example had little to no crimp and a very smooth surface.The minimum distance is process dependent on the properties andcharacteristics of the fibrous layers and the particular needlingprocess to be used in forming the fibrous preform structure. The minimumdistance is established empirically.

Transport depth must be known with some degree of certainty in order toquantify permanent fiber transport. As depicted in FIG. 3, fibertransport depth is determined relative to the top layer 40a. Referringagain to FIG. 2, the fiber transport depth is preferably determinedrelative to an estimated surface position 92 of the exposed surface 44beneath the needles 14 during the needling process. The exposed surface44 moves away from the needles during the needling pass, due at least inpart to compaction of the top layer 40a, as the fibrous structure passesthrough the needling zone 32. According to a preferred embodiment, theestimated surface position 92 for each needling pass is determined bydetermining a pre-needled surface position 94 of the exposed surface 44,determining a post-needled surface position 96 of the exposed surface44, and determining the estimated surface position 92 during needling byaveraging the pre-needled surface position 94 and the post-needledsurface position 96.

The pre-needled surface position 94 can be actively determined, asdepicted in FIG. 1, by a first transducer 98 having a surface followingdevice 100 that tracks the position of the exposed surface 44 before thefibrous preform structure is subjected to needling beneath the feltingneedles 14. The post-needled surface position can be actively determinedby a second transducer 102 having a surface following device 104 thattracks the position of the exposed surface 44 after the fibrous preformstructure is subjected to needling beneath the felting needles 14.Surface position information from the first and second transducers 98and 102 is transmitted to controller 28 via transducer lines 106 and108. The controller 28 then processes the signals and determines theestimated surface position at each point in the process.

The pre-needled surface position 94 and post-needled surface position96, and hence the estimated surface position 92, may also be determinedby previously forming a substantially similar fibrous preform structurein a substantially similar process and determining the pre-needledsurface position 94 and the post-needled surface position 96 duringformation of the substantially similar fibrous preform structure. Thisaspect will be discussed in more detail with respect to FIGS. 6 and 8.

Referring now to FIGS. 5A-5D, a process is depicted according to anaspect of the invention for forming a fibrous preform structure bycohering several fibrous layers together beginning with two fibrouslayers 108 and 110 to which additional fibrous layers 112, 114 and 116are added. As depicted in FIG. 5A, the needling process begins with twolayers 108 and 110 that are disposed on the support 12 beneath themultitude of felting needles 14. Each of layers 108, 110, 112, 114, and116 comprise three unidirectional sub-layers of OPF fiber lightlyneedled together into a coherent layer, as previously described. Thesupport 12 is formed from metal and perforated with holes into whichneedles 14 may penetrate. Needles 14 are shown in FIGS. 5A-5D at theirfurthest downward travel position. In FIG. 5A, fiber is permanentlytransported from layer 110, through layer 108, and into the support 12in a first needling pass as the support 12 is driven in the direction ofarrow 118. A perforated support such as support 12 does not grip thetransported fiber and layers 108 and 110 are not significantly compactedby the needling process. Thus, the pre-needled surface position 94 andthe post-needled surface position 96 are about the same. This was asurprising discovery.

In FIG. 5B, an additional fibrous layer 112 is added and needled tolayers 108 and 110 in a second needling pass as the support is driven inthe direction of arrow 120. At this point, layers 108 and 110 begin tocompact and pre-needled surface position 94 is above the post-needledsurface position 96, resulting in estimated surface position 92. Anotherfibrous layer 114 is added in FIG. 5C and needled to layers 108, 110 and112 in a third needling pass as the support is driven in the directionof arrow 122. Layer 114 is compacted and layers 108, 110 and 112 arecompacted some more. In FIG. 5D, another fibrous layer 116 is added andneedled to layers 108, 110, 112, and 114 in a fourth needling pass asthe support is driven in the direction of arrow 124. Top layer 116 iscompacted, and layers 108, 110, 112, and 114 experience more compaction.Thus, the exposed surface into which the needles are driven moves awayfrom the needles during the needling process due at least in part tocompaction in the top layer, and due at least in part to compactionwithin the stack of layers beneath the top layer. In some needlingprocesses, the direction in which the support is driven alternates fromone needling pass to the next. Any such variation is considered to bewithin the purview of the invention.

An example of a relationship between surface position and needling passthat represents the FIGS. 5A-5D process is presented in FIG. 6. Needlingpass 1 represents FIG. 5A where two layers are disposed beneath thefelting needles, and the pre-needled surface position is nearly the sameas the post-needled surface position. Another layer is added before eachneedling pass beginning with needling pass 2. Needling pass 2 representsFIG. 5B where three layers are disposed beneath the felting needles. Asshown in FIG. 6, these layers are beginning to compact. Additionallayers are added in needling passes 3 and 4 which correspond to FIGS. 5Cand 5D. Data from the addition of two layers in needling passes 5 and 6are also shown. The estimated surface position for each needling step isdepicted, which is essentially the average between the pre-needledsurface position and post-needled surface position for each needlingpass. A compaction factor, F, is also depicted for each needling pass.The compaction factor represents an offset from the post-needledthickness for any given needling pass and establishes the estimatedsurface position relative to the post-needled surface position. Thus,compensation for compaction in the top layer and the lower layersappears in the compaction factor. According to a preferred embodiment,the compaction factor for a given needling pass is calculated bysubtracting the post-needled thickness of the fibrous structure from thepre-needled thickness of the fibrous structure and dividing by two. Thecompaction factor may be used in a process that characterizes ormanipulates the Z-fiber distribution throughout the thickness of afibrous preform structure. This aspect of the invention will bediscussed more fully in relation to FIG. 11 and Equations 7 and 8.

Another process that also represents an aspect of the invention isdepicted in FIGS. 7A-7D. The multitude of felting needles 14 are shownat their furthest downward travel in FIGS. 7A-7D. In FIG. 7A, a layer128 is disposed over support 12 beneath the multitude of felting needles14. The support 12 is formed from metal and perforated with holes intowhich needles 14 may penetrate. Another layer 130 is disposed over layer128. Layer 130 comprises three unidirectional sub-layers of OPF fiberneedled together into a coherent layer, as previously described. Layer128 is an 800 g/m² pre-needlepunched airlaid OPF web as described inEuropean Patent Application 0 530 741 A1, to Morris et al. Layer 128 isabout 8-13 millimeters thick and layer 130 is about 3 millimeters thickbefore the first needling pass of FIG. 7A. Thus, the pre-needledthickness of layer 128 is much greater than the pre-needled thickness oflayer 130. Layer 130 is needled to layer 128 in a first needling pass assupport 12 is driven in the direction of arrow 138.

Referring still to FIG. 7A, the layers 128 and 130 are compacted asignificant amount during the first needling pass (in contrast to layers108 and 110 of FIG. 5A) resulting in a large change from the pre-needledsurface position 94 to the post-needled surface position 96. Thecompaction in layer 128 during the first needling pass is due to severalfactors. Layer 128 is formed of short fibers having a mean length of 25millimeters or less when measured according to ASTM D 1440, which do notexhibit a great deal of resilient behavior when subjected to needling.In other words, any fiber transported from or within layer 128 byfelting needles 14 is permanently transported since the fibers are shortand have little tendency to pull back to their starting positions. Layer128 is also thick enough to permit permanent fiber transport originatingand ending entirely within the layer. Also, layer 128 is thick enough togrip fibers transported from layer 130. Finally, layer 128 is moresusceptible to compaction since it is thick and of lesser fiber volume(fiber per unit volume), and has not previously been subjected to agreat amount of needling.

An additional layer 132 is added in FIG. 7B which is compacted during asecond needling pass as support 12 is driven in the direction of arrow140. Layers 128 and 130 are further compacted, such that the combinedcompaction of the layers results in a change from pre-needled surfaceposition 94 to post-needled surface position 96, and an estimatedsurface position 92. Additional layers 134 and 136 are added and needledin third and fourth needling passes as support 12 is driven in thedirection of arrows 142 and 144, respectively, as depicted in FIGS. 7Cand 7D. These needling passes cause further compaction of previouslyneedled layers. Once again, the exposed surface into which the needlesare driven moves away from the needles during the needling process dueat least in part to compaction in the top layer, and due at least inpart to compaction within the stack of layers beneath the top layer.

An example of surface position versus needling pass that represents theFIG. 7A-7D process is depicted in FIG. 8. Needling pass 1 representsFIG. 7A where two layers are disposed beneath the felting needles. Asshown in FIG. 8, needling pass 1 induces a relatively large change frompre-needled surface position to post-needled surface position. Anotherlayer is added before each needling pass beginning with needling pass 2.Needling pass 2 represents FIG. 7B where three layers are disposedbeneath the felting needles. As shown in FIG. 8, the change frompre-needled surface position to post-needled surface position is lessthan the previous needling step. Additional layers are added in needlingpasses 3 and 4 which correspond to FIGS. 7C and 7D. Data from theaddition of six layers in needling passes 5-10 are also shown. Theestimated surface position for each needling step is depicted, which isessentially the average between the pre-needled surface position andpost-needled surface position for each needling pass. Compaction factorF is also depicted. Note that the FIG. 8 compaction factor has asignificantly different trend from the FIG. 6 compaction factor.

Experiments have shown that estimated surface position versus needlingstep and compaction factor as depicted in FIGS. 6 and 8 does not changesignificantly from one fibrous preform structure to the next, as long asthe fibrous preform structures are substantially similar and are formedin substantially similar processes. Therefore, the estimated surfaceposition and compaction factor for each needling pass may be derivedfrom a previously established relationship. In such case, therelationship is established by previously forming a substantiallysimilar fibrous preform structure in a substantially similar process anddetermining the estimated surface position during formation of thesubstantially similar fibrous preform structure. FIGS. 6 and 8 representsuch previously established relationships that can be subsequently usedin the production of other substantially similar fibrous preformstructures.

Variations in compaction can arise from different sources. FIGS. 5 and 6involve a situation where the first two fibrous layers do notsignificantly compact during the first needling pass. The compaction inthese layers occurs during subsequent needling passes. FIGS. 7 and 8involve a situation where one of the beginning layers is relativelythick and is compacted during the first needling pass, but continues tocompact during subsequent needling passes. The process according to theinvention is flexible enough to address both of these situations, andapplication of the invention is not limited to these examples.Compaction can occur in different ways depending on the characteristicsof the fibrous layers and the particular needling process and machinery,and may be accounted for according to the principles provided by thisdisclosure. Further, FIGS. 5 through 8 were derived from fibrous preformstructures formed on a perforated bedplate that does not grip thetransported fibers, as previously described. A support formed fromfoamed plastic or upright brush bristles may grip the transported fibersmore effectively. However, some compaction effects could still occur andmay be addressed according to the principles provided by thisdisclosure. Any such variations are considered to be within the purviewof the invention.

According to another aspect of the invention, permanent fiber transportfrom each layer in the set of layers may be quantified. However,identifying the exact measure of fiber transported from each layer isnot necessary in the practice of the invention. What is meant by "exactmeasure" is identifying for each layer a certain mass of fiberspermanently transported from that layer, or number of fibers transportedfrom that layer, or similar quantification. According to the inventionthere is provided a technique for generating a relative comparison oftransport efficiency and resultant Z-fiber generation for each layer ofthe fibrous preform structure, or at each stage of the process. Thisrepresents a tremendous advantage since tracking and identifying fiberloading and unloading in a particular barb as it passes through thelayers is presently extremely difficult. For example, with the fibrouslayers and needling processes discussed herein for the purpose ofdescribing the present invention, the barbs on a given needle becomecompletely loaded with fiber almost immediately after penetrating theexposed surface. The barbs unload to some extent as they penetrate intolower layers due to fiber breakage. The barbs engage more fiber fromwhatever layer they happen to be passing through as they unload. Thus,most of the fiber in Z-fiber bundle 66 of FIG. 3 is from layer 40a, andsmaller fractions are from layers 40b and 40c. Identifying the exactmeasure of fiber in a Z-fiber bundle from a given layer is desirable,but not necessary in the practice of the invention, as long as the fiberpermanently transported from a given layer is quantified in some manner.

According to an aspect of the invention, permanent fiber transport isquantified as follows. Referring again to FIG. 3, each barb engages anamount of fiber from a given layer in the set of layers as each barbpasses through that layer. The amount is usually different for each barbon a given felting needle. Each felting needle 14 engages and transportsa quantity of fiber from a given layer in the set of layers, which isthe sum of the amount engaged from that layer by each barb that passesthrough that layer. For example, in a certain preferred embodiment, thetop layer in each needling pass comprises three cross-lappedunidirectional sub-layers of OPF fiber needled together into a coherentlayer, as previously described, and the felting needles are cataloguenumber 15×18×36×3.5 C333 G 1002 needles, available from Groz-Beckert ofGermany. In this example, the first barb 68 engages 70% of the quantitytransported from a given layer, the second barb 70 engages 25% of thequantity transported from a given layer, and the third barb 72 engagesonly 5% of the quantity transported from a given layer during a givenneedling pass. Based on current understanding, tests performed by theneedle manufacturer have shown that other barbs spaced further up thefelting needle do not engage and transport fiber as effectively in thisprocess. Also, most of the fiber transported by a given felting needlein this process appears to be from the top layer since each barb almostimmediately loads with fiber upon being driven into the top layer. Thebarbs tend to unload and pick up new fiber from other layers as theypass through the fibrous structure, as previously described in relationto FIG. 3. However, this may not be the case for all needling processesor for all types of fibrous layers. The fiber transport characteristicsdepend on the characteristics of the fibrous layers and the particularneedling process and machinery, and should be empirically determined foreach system.

The quantity of fiber permanently transported from a given layer, toplayer 40a for example, is approximated by summing together the amountengaged from that layer by each barb that travels at least the minimumdistance from that layer. For example, barb 68 travels through thetransport distance 52 from the top layer 40a, which is greater than theminimum distance 50 from that layer. Therefore, any fiber transported bybarb 68 from layer 40a is permanently transported. Barb 70 travelsthrough a distance from top layer 40a as calculated by the followingequation:

    D1.sub.2 =D1.sub.1 -d.sub.1                                Eqn. (1)

wherein D1₂ is the distance second barb 70 travels from layer 40a, D1₁is the distance first barb 68 travels from top layer 40a (D1₁ =transportdepth 52), and d₁ is the distance 71 between the first barb and thesecond barb 70. D1₂ is also greater than the minimum distance 50 fromlayer 40a, as depicted in FIG. 3, which means that any fiber that secondbarb 70 transports from layer 40a is permanently transported. Barb 72travels through a distance from top layer 40a as calculated by thefollowing equation:

    D1.sub.3 =D1.sub.1 -d.sub.1 -d.sub.2                       Eqn. (2)

wherein D1₃ is the distance third barb 72 travels from layer 40a, D1₁ isthe distance first barb 68 travels from top layer 40a (D1₁ =transportdepth 52), d₁ is the distance 71 between the first barb and the secondbarb 70, and d₂ is the distance 73 between the second barb 70 and thethird barb 72. D1₃ is greater than the minimum distance 50 from layer40a, which means that any fiber that third barb 72 transports from layer40a is permanently transported. Therefore, 100% of the fiber engaged andtransported by the needle from layer 40a by felting needle 14 ispermanently transported since D1₁, D1₂, and D1₃ are all greater than theminimum distance 50. This example is based on the previously statedpartial quantity estimates of 70% for the first barb, 25% for the secondbarb, and 5% for the third barb.

Similar calculations may be performed for layer 40b, wherein D2₁ istransport depth 54, according to the following equations:

    D2.sub.2 =D2.sub.1 -d.sub.1                                Eqn. (3)

    D2.sub.3 =D2.sub.1 -d.sub.1 -d.sub.2                       Eqn. (4)

wherein D2₁ is the distance barb 68 travels from layer 40b, (D2₂=transport depth 54), D2₂ is the distance second barb 70 travels fromlayer 40b, D2₃ is the distance third barb 72 travels from layer 40a, d₁is the distance 71 between the first barb and the second barb 70, and d₂is the distance 73 between the second barb 70 and the third barb 72.Performing these calculations for this example would show that all threebarbs traveled more than the minimum distance from layer 40b, meaningthat 100% of the fiber engaged and transported by the needle from layer40b is permanently transported. Equations 1-4 are defined by thefollowing equation: ##EQU1## wherein N specifies a given layerpenetrated by the first barb 68 (N=1 for the top layer 40a, N=2 forlayer 40b, N=3 layer 40c, . . . ), B indicates a specific barb on thefelting needle that transports fiber (B=2 for second barb 70, B=3 forthird barb 72, and so on up the needle), DN_(B) is the distance aspecific barb travels relative to layer N, and d_(b-1) is the distancefrom one barb (b-1) to the next barb (b) along the needle. Thus, thecalculations can be performed for as many layers as are penetrated bythe first barb, and for every barb on a felting needle that engages andtransports fiber. However, there is no need to perform the calculationsfor more than the set of layers since fiber is not permanentlytransported from more than the set of layers. Equation 5 applies only ifB≧2 because there is no need to determine transport depth for additionalbarbs if there is only one barb on the needle.

Carrying out the calculations for layer 40c (n=3) using transport depth56 for D3₁ would show that D3₁ and D3₂ are greater than the minimumdistance 50, but D3₃ is less than the minimum distance. Therefore, fiberengaged by third barb 72 from layer 40c would not be permanentlytransported, but fiber engaged by first barb 70 and second barb 72 fromlayer 40c would be permanently transported. Only 95% of the fiberengaged from layer 40c by the needle was permanently transported (70%for the first barb+25% for the second barb). The 5% of fiber engaged bythird barb 72 from layer 40c was not permanently transported. Aspreviously described, these estimates of permanent fiber transportdepend on the particular felting needle, fibrous layer characteristics,and needling process. The percentages of 70%, 25% and 5% are used by wayof example and are not intended to limit the invention to just thesequantities.

It is important to note that permanent fiber transport from each layeris thus quantified without identifying the exact measure of permanentfiber transport from each layer. The quantity of permanent fibertransport in this context refers to the percentage (or any equivalentmeasure such as a fraction) of fiber engaged by the needle from a givenlayer at the bottom of the needle stroke that becomes permanentlytransported. It is quite foreseeable that the quantity of permanentfiber transport may eventually become quantifiable in other ways. Forexample, using the exact measure for each layer would be desirable if aneffective technique for determining the exact measure for each layerbecomes available.

Still referring to FIG. 3, the transport depths 54, 56, 58, 60, 62, and64 from each layer 40a through 40f may be derived from the transportdepth 52 relative to top layer 40a. According to a preferred embodiment,the transport depth 52 is determined relative to the estimated surfaceposition of the top layer 40a, as previously described. More preferably,the estimated surface position is an average of the pre-needled surfaceposition and post-needled surface position. A post-needled thickness ofeach layer disposed beneath the felting needles is then determined. Thetransport depth for a given layer is determined from transport depth 52by subtracting the thickness of each layer disposed above that layerfrom the transport depth 52. For example, transport depth 54 from layer40b is determined by subtracting the thickness of layer 40a fromtransport depth 52. The transport depth 56 relative to layer 40c isdetermined by subtracting the post-needled thicknesses of layers 40a and40b from transport depth 52. Thus, the transport depth relative to anygiven layer may be calculated from the transport depth 52 from the toplayer 40a according to the following equation: ##EQU2## wherein Nspecifies a given layer beneath top layer 40a (N=1) penetrated by thefirst barb 68 (N=2 for the second layer 40b, N=3 for the third layer40c, N=4 for the fourth layer 40d, . . . ), DN₁ is the transport depthfrom layer N, D1₁ is the transport depth from the top layer 40a(transport depth 52), and t_(n-1) is the thickness of the each layerdisposed above layer N. The calculations may be repeated for as manylayers as are penetrated by the first barb 68 (seven layers in FIG. 3).However, there is no need to perform the calculations for more than theset of layers (layers 40a-40c in FIG. 3) since fiber is not permanentlytransported from more than the set of layers. This equation is based onthe distance the first barb 68 travels relative to the top layer 40a.The distance that additional barbs travel relative to each layer may becalculated according to Equations 1-5.

Alternatively, the transport depth of the first barb from each layer maybe calculated as follows: ##EQU3## wherein the variables are the same asin Equation 6, and F is the compaction factor for that needling pass.This equation calculates transport depth from the sub layers relative tothe post-needled position of the exposed surface of the fibrous preformstructure. Equation 6 provides an acceptable estimate of transportdepth, but Equation 6a is more accurate, especially with a top fibrouslayer that has a relatively large compaction factor. The 800 g/m²airlaid web, as previously described, is an example of a fibrous layerhaving a relatively large compaction factor.

If the layers are substantially similar, the post-needled thickness maybe calculated for each needling pass as an average thickness of thegroup of layers comprising the fibrous structure at that needling pass.The thickness of the group of layers may change as the layers aresubjected to additional needling passes and as additional layers areadded as depicted in FIGS. 5A-5D. An example of a relationship betweennumber of layers and average layer thickness is depicted in FIG. 9,beginning with two layers. FIG. 9 is referred to as a compaction curve.A layer was added to the fibrous preform structure before each needlingpass. As shown, average thickness decreases as the number of layers (andneedling passes) increases. This trend is caused by the fact thatpreviously needled layers comprising the fibrous structure continue tocompact for several subsequent needling passes. These layers compact asadditional layers are added.

The average thickness of layers comprising the fibrous structure may bedetermined at each point in the process from a curve such as thatpresented in FIG. 9 by previously forming a substantially similarfibrous preform structure in a substantially similar process anddetermining the average thickness during formation of the substantiallysimilar fibrous preform structure. This information can then be used toform subsequent fibrous preform structures without actively determiningthe average layer thickness during the process. This approach greatlysimplifies the process. A curve such as that presented in FIG. 9 may beprogrammed into a controller such as controller 28 of FIG. 1.

The thickness of the individual layers may be used rather than theaverage layer thickness in the practice of the invention. The thicknessof layer 128 for a series of needling passes (as shown in FIGS. 7A-7D)was determined by measuring a sliced-off portion of the fibrousstructure following each needling pass, and is depicted in FIG. 10 ascurve I beginning with the pre-needled thickness (before needling pass1). Curve II represents the thickness of an airlaid web such as layer128 when needled to a substantial fibrous structure at a later point inthe process. Note that the airlaid web compacts much differentlydepending on the point in the process at which the layer is applied.

As shown in this FIG. 10, compaction of a thick layer such as layer 128can continue for several needling passes. Failing to account for thiscompaction can cause a significant deviation from a desired Z-fiberdistribution through the thickness of the fibrous preform structure.Characterizing individual layers is necessary if any of the layers aresignificantly different from other layers within the fibrous structure.Once again, a curve such as FIG. 10 may be determined duringconstruction of a previously formed fibrous preform structure and usedduring formation of subsequently formed fibrous preform structures thatare substantially similar. The curve should not change a significantamount from formation of one fibrous preform structure to the next aslong as the processes are substantially similar.

More than one needling pass may permanently transport fiber from a givenlayer in the set of layers. Therefore, a technique is desired whereby acumulative quantity of fiber permanently transported from a given layermay be determined. According to an aspect of the invention, each barbengages an amount of fiber from a given layer in the set of layers aseach barb passes through that layer during a given needling pass. Aquantity of fiber permanently transported from a given layer during theneedling pass is approximated by summing each amount engaged from thatlayer by each barb that travels at least the minimum distance from thatlayer, as previously described. A cumulative quantity of fiberpermanently transported from a given layer is approximated by summingthe quantity from each needling step that permanently transports fiberfrom the layer. The cumulative quantity of fiber permanently transportedfrom a given layer is conveniently approximated using a table such asthat depicted in FIG. 11.

In FIG. 11, needling pass is numbered along a vertical axis on the leftside of the table. The layers comprising the fibrous structure for eachneedling pass are numbered along a horizontal axis at the top of thetable. A total of 32 layers were needled together, beginning with layers1 and 2 at needling pass 1. Layers 1 and 32 were 800 g/m² airlaid weband layers 2 through 31 each comprised three cross-lapped unidirectionalsub-layers of OPF fiber needled together into a coherent layer, aspreviously described. A layer was added before each needling pass fromneedling pass 2 to needling pass 31, followed by three walkout needlingpasses WO1, WO2, and WO3 during which the fibrous structure was loweredand needled without adding layers. The bedplate position relative to theinitial bedplate position at needling pass 1 is designated as "δ" in thefirst column on the right of the table. The change in bedplate positionfor each needling pass from the previous needling pass is shown in thecolumn labeled "δ^(i) -δ^(i-1)." As depicted in FIG. 2, the multitude offelting needles 14 were reciprocally driven through a fixed range oftravel 160, and the fibrous structure was disposed on the bedplate 12and moved in the direction of arrow 34. The vertical position ofbedplate 12 was controlled such that moving the bedplate 12 toward themultitude of felting needles increased fiber transport depth and movingthe bedplate away from the multitude of felting needles decreased fibertransport depth.

The total thickness of the fibrous structure after each needling pass isshown in the column labeled "T". A thickness of layer 1 (the airlaidweb) was determined for each needling pass and is shown in the columnlabeled "t_(air) ", and was determined for each needling pass bymeasuring sliced-off portions of the fibrous preform structure for eachneedling pass. It could also be determined from a previously establishedrelationship as previously described in relation to FIG. 10. An averagelayer thickness t_(ave), after needling, was determined for eachneedling pass by dividing T by the number of layers comprising thefibrous structure after subtracting the thickness of layer 1 (theairlaid web). The average layer thickness t_(ave), is shown in thecolumn labeled "t_(ave)." The transport depth D1₁ of the first barbrelative to the top layer for each needling pass is shown in the columnlabeled "Actual D1₁ " and was calculated according to the followingequation:

    D1.sub.1 =P.sub.0 -c+T+F-δ                           Eqn. (7)

wherein D1₁ is the transport depth of the first barb relative to the toplayer, δ is the bedplate position relative to the initial bedplateposition (a positive δ indicates a movement away from the needles),P_(o) is the initial needle penetration depth when δ=0, T is totalfibrous structure thickness after each needling pass, c is the distancebetween the first barb and the tip of the felting needle, and F is thecompaction factor. FIG. 11, all lineal dimensions are in millimeters.Referring to FIG. 12, P_(o) is defined as the distance between the tipof the needle and the top of the bedplate when δ=0 at the first needlingpass. P_(o) is positive when the tip of the needle is below the support,as shown in FIG. 12, and negative when the tip of the needle is abovethe support. The distance c between the tip of the needle and the firstbarb is also depicted, as well as the total post-needled thickness ofthe fibrous structure T.

The compaction factor, F, was determined from FIG. 8 for each needlingpass. Thus, the post-needled surface position for a given needling passis determined from the bedplate position δ and post-needled thickness T,and the estimated surface position during needling is determined byadding the compaction factor F to that position.

For this needling process, the minimum distance to achieve permanentfiber transport was about 7 mm as determined according to the processdescribed in relation to FIG. 4. Therefore, all three barbs had totravel at least 7 mm from the upper boundary of a given layer in orderto permanently transport fiber engaged and transported from that layerby all three barbs. With the felting needle used in this process, thefirst barb is spaced 1.06 mm from the second barb, and the third barb isspaced 1.06 mm from the second barb. Therefore, the first barb had totravel at least 9.12 mm (7 mm+1.06 mm+1.06 mm) past the upper boundaryof a given layer in order to achieve permanent transport of 100% of thefiber engaged by all three barbs from a given layer by the feltingneedles. In FIG. 11, it was assumed that the first barb transported 70%of the fiber, the second barb transported 25% of the fiber, and thethird barb transported 5% of the fiber, as previously described. At thispoint, equations 1-6 could be used to calculate the quantity ofpermanent fiber transport from each layer for a given needling pass.However, the following table may be utilized to reduce the number ofcalculations by focusing on the transport depth of the first barbrelative to each layer:

                  TABLE 1                                                         ______________________________________                                        % Permanent Transport                                                                             Transport Depth                                           From a Given Layer n                                                                              of First Barb                                             ______________________________________                                        100                 Dn.sub.1  ≧ 9.12                                   95                  9.12 > Dn.sub.1  ≧ 8.06                            70                  8.06 > Dn.sub.1  ≧ 7.00                            25                  7.00 > Dn.sub.1  ≧ 6.50                            0                   Dn.sub.1  < 6.50                                          ______________________________________                                    

wherein Dn₁ is calculated for each layer according to Equation 6. Table1 eliminates the need to calculate the transport depths for eachindividual needle barb. In Equation 6, the average layer thicknesst_(ave) at a given needling pass was used for t_(n) at that needlingpass if layer n was a crosslap layer, and t_(air) was used for t_(n) iflayer n was an airlay layer at each needling step.

Referring still to Table 1 and the first range of transport depth(Dn≧9.12), all three barbs penetrate far enough such that 100% of thefiber is permanently transported (70%+25%+5%). In the second range(9.12>Dn≧8.06), only 95% of the fiber is permanently transported becauseonly the first and second barbs penetrate far enough to permanentlytransport fiber. In the third range (8.06>Dn≧7.00), only 70% of thefiber is permanently transported because only the first barb penetratesfar enough to permanently transport fiber. These ranges are readilydeduced from the previously disclosed needling conditions and fibrouslayer materials of the FIG. 11 fibrous preform structure. The fourthrange of 25% permanent transport reflects an in-between range where thetop layer is partially, but not fully, tacked to the adjacent layeraccording to the FIG. 4 process. As noted in relation to FIG. 4, the toplayer begins to tack at a transport depth of about 6.5 mm and fullytacks at about 7.0 mm. Thus, the transition from no tack to full tackfor this process appears to occur with an increase of 0.5 mm intransport depth. Including the 25% range provides a lower increment inquantity of permanent fiber transport that reflects this transitionrange. A table such as Table 1 could be constructed for any needlingprocess and type of fibrous layer. However, the invention is not limitedto the Table 1 ranges since these principles may be applied to nearlyany fibrous layer material and needling process.

Determining a cumulative quantity of permanent fiber transport accordingto the FIG. 11 process is demonstrated by the following example. In FIG.11, P_(o) is 10.60 mm and c is 6.36 mm. Needling pass number 5 has atransport depth D1₁ from the top layer of 13.51 mm as calculatedaccording to Equation 7 (10.6 mm-6.36 mm+12.07 mm+1.0 mm-3.8 mm), whichis greater than 9.12 mm which means that 100% of the fiber transportedfrom the top layer, which is layer 6 for needling pass 5, is permanentlytransported. Thus, a "100" appears in layer 6 for needling pass 5. Thetransport depth D2₁ from layer 4 is 11.81 mm (13.51 mm-1.70 mm), whichis greater than 9.12 mm which means that 100% of the fiber transportedfrom layer 5 is permanently transported during needling pass 5. Thus, a"100" appears in the layer 5 column of needling pass 5. The transportdepth D3₁ from layer 4 is 10.11 mm (13.51 mm-1.70 mm-1.70 mm), which isgreater than 9.12 mm which means that 100% of the fiber transported fromlayer 4 is permanently transported during needling pass 5. Thus, a "100"appears in the layer 4 column of needling pass 5. The transport depthD4₁ from layer 3 is 8.41 mm (13.51 mm-1.70 mm-1.70 mm-1.70 mm), which isless than 9.12 mm but is greater than 8.06 mm which means that 95% ofthe fiber transported from layer 3 is permanently transported duringneedling pass 5. Thus, a "95" appears in the layer 3 column of needlingpass 5. The transport depth D5₁ from layer 4 is 6.71 mm (13.51 mm-1.70mm-1.70 mm-1.70 mm-1.70 mm), which is less than 7.0 mm but greater than6.5 mm which means that 25% of the fiber transported from layer 2 ispermanently transported. Thus, there were five layers in the set oflayers at needling pass 5 (layers 2-6). These calculations are repeatedfor all the needling passes and the quantities of transport for eachneedling pass are entered into the table as described. After doing so, acumulative quantity of fiber transport for each layer is calculated bysumming all the quantities of permanent transport appearing in thecolumn for each layer. For example, layer 4 is subjected to permanenttransport quantified as 100 during needling pass 3, 100 during needlingpass 4, 100 during needling pass 5, 95 during needling pass 6, and 25during needling pass 7 for a total of 420 that appears at the bottom ofthe matrix. The sum of the quantities of permanent fiber transport foreach layer is called the cumulative quantity of permanent fibertransport (CQT). The CQT quantifies total permanent fiber transport fromeach layer when fiber is permanently transported from a layer by atleast two needling passes.

The thickness of layer 1 was derived from Curve I of FIG. 10 and thethickness of layer 32 was derived from Curve II of FIG. 11 since boththese layers were airlaid web. Note that these layers compactdifferently since they are added at different points in the process.Thus, the invention is flexible and able to compensate for variations incompaction characteristics that occur throughout the process.

The CQT for all the layers appears in a row along the bottom of the FIG.11 matrix labeled "Actual CQT." Note that the CQT generally decreasesfrom a high of 465 at layer 3 to a low of 200 at layer 32. When forminga brake disk, several layers are removed during and after thedensification process resulting in two opposing wear surfaces spacedfrom each other. The CQT at both wear surfaces (WS) is presented in FIG.11. The CQT at one surface is about twice the CQT at the other wearsurface (420/220). This non-uniformity has been verified by interlaminarpeel tests of the fibrous preform structure characterized by FIG. 11.The force necessary to peel layers apart decreases as CQT decreaseswhich follows from the fact that the CQT quantifies the amount ofZ-fiber permanently transported. A higher CQT indicates a higherquantity of Z-fiber, and Z-fiber is responsible for cohering the layers.Thus, more Z-fiber equates to a higher interlaminar peel force.

The process may be carried a step further wherein the needling processis manipulated to generate a chosen CQT for each layer. Choosing the CQTfor each layer comprising the fibrous preform structure is a matter ofpreform design according to desired properties of the final fibrouspreform structure, and is not part of the invention. Equation 8 asfollows may be used to achieve a desired quantity of permanent transportfor each layer:

    δ.sup.i =P.sub.0 -c+T.sup.i-1 +W.sup.i -D1.sub.1.sup.i Eqn. (8)

wherein δ^(i) is δ for the current needling pass I, T^(i-1) is the totalthickness of the fibrous structure from the previous needling pass I-1,W^(i) is a prediction factor, and D1₁ ^(i) is the desired transportdepth D1₁ for the current needling pass. The prediction factor W^(i) isthe sum of the projected thickness of the top layer following thecurrent needling pass and the projected compaction factor for thatneedling pass.

Two approaches are possible using Equation 8 to achieve a desired CQTfor each layer in a fibrous preform structure. In some cases, thetransport depths D1₁ ^(i) for each needling pass may be known that willgenerate a desired CQT for each layer. Matching the transport depths D1₁^(i) during construction of a similar preform structure with the knowntransport depths D1₁ ^(i) may generate a similar CQT for each layer.Establishing known transport depths that will generate a desired CQT foreach layer may be accomplished, for example, by using Equation 7 and theprinciples discussed thus far in relation to the invention tocharacterize a fibrous preform structure formed by a particular processand to determine a transport depth for each needling pass, and aresulting CQT for each layer. A process for characterizing D1₁ for eachneedling pass and a CQT for each layer in a preform structure waspreviously described with great detail in relation to FIG. 11, and usingEquation 7. An example of such information that may be obtained from apreviously constructed fibrous preform structure is presented in the"Desired D1₁ " column and the "Desired CQT" row of FIG. 11.

A preform structure having "Actual D1₁ " transport depths and "ActualCQT" quantities substantially similar to the "Desired D1₁ " transportdepths and "Desired CQT" quantities may be constructed as follows.Equation 8 is used to calculate δ^(i) for each needling pass. First, theprediction factor W^(i) must be determined. According to a preferredembodiment, W^(i) for a given needling pass (I) is derived from dataobtained from the previous needling pass (I-1). More preferably, W^(i)is derived by summing t_(ave) ^(i-1) and F^(i-1) if the current layer(I) is substantially the same as the previous layer (I-1).Alternatively, layer thickness and F for a given needling pass may bederived by using curves such as those presented in FIGS. 8 and 10,especially if the layers are not substantially similar. Referring againto FIG. 11 and using needling pass 5 as an example, data from needlingpass 4 was used to determine a W⁵ of 2.91 mm for needling pass 5 bysumming F⁴ (1.0 mm) and T_(ave) ⁴ (1.91 mm). Thus, δ⁵ is 3.8 mm (10.6mm-6.36 mm+11.18 mm+2.91 mm-14.53 mm) for needling pass 5, according toEquation 8, and using the desired transport depth of 14.53 mm at needlepass 5 for D1₁ ⁵. The support is then adjusted to a δ⁵ of 3.8 mm and thefibrous structure is subjected to needling pass 5.

Equation 8 may thus be used to calculate δ^(i) for each needling pass.The support is adjusted to that δ^(i) and the fibrous structure issubjected to that needling pass. After a given needling pass isperformed, the fibrous layer thicknesses comprising the fibrousstructure for that needling pass are determined, as previouslydescribed, and Equation 7 is used to establish the actual transportdepth for that needling pass which serves to verify that the process ison track. The actual transport depth calculated for each needling passappears in the column labeled "Actual D1₁ ", which has already beendescribed in great detail.

Actual CQT versus desired CQT for this process is shown at the bottom ofFIG. 11. The actual CQT is preferably within ±10% of the desired CQT ateach needling pass. Constructing a preform according to this processallows the process to actively correct itself for any variations thatoccur during the process, and results in an actual Z-fiber distributionthat closely approximates the desired Z-fiber distribution. Variationsmay arise from compaction and subtle changes in fibrous layer thicknessduring the process, and other sources.

According to another approach that also represents an aspect of theinvention, the quantity of permanent fiber transport of each layer maybe manipulated at each needling pass to achieve a desired CQT for eachlayer. Referring to FIG. 13, a table is presented that was used toconstruct a fibrous preform structure having a substantially constantCQT value through a number of adjacent layers. In this example, a CQTvalue of 270 was desired for each layer, which appears in a row alongthe bottom of the table. The actual CQT for each layer is also depicted,and closely follows the desired CQT. Thirteen layers were needledtogether, each layer being comprised of three cross-lappedunidirectional sub-layers of OPF fiber needled together into a coherentlayer, as previously described. The quantity of permanent transport fromeach layer for each needling pass was manipulated as follows.

The process began by needling layers 1 and 2 together by subjectingthose layers to needling pass 1. Equation 7 was then used withappropriate measurements of the fibrous preform structure to calculatethe actual D1₁ of 11.12 mm for needling pass 1, which appears in thefurthest column to the right of FIG. 13. In this process, P_(o) was 11.0mm and c was 6.36 mm. The initial bedplate setting for needling pass 1is a matter of judgment, and should be sufficient to cohere the layers.The initial bedplate position δ¹ preferably results in an initialtransport depth D1₁ ¹ that is reasonably deep enough to achieve 100%transport from layers 1 and 2 during needling pass 1 withoutover-needling the layers. As before, the column labeled "T" is thepost-needled thickness of the preform, and the column labeled "t_(ave) "is the average layer thickness calculated by dividing T by the number oflayers comprising the fibrous preform structure at each needling pass.The compaction factor, F, is derived from FIG. 6 and also appears inFIG. 13.

Beginning with needling pass 2, a desired transport depth D1₁ ² isdetermined that is sufficient to achieve a desired quantity of permanentfiber transport from each layer in a set of layers. The set of layersand the quantity for each layer are determined at each needling step asnecessary to generate the desired CQT for each layer. At needling pass2, for example, permanent transport from layer 3 should be 100% sincelayer 3 has not yet been needled and a CQT of 270 is desired, permanenttransport from layer 2 should be 100% since layer 2 has a CQT of only100 after needling pass 1 and a CQT of 270 is desired, and permanenttransport from layer 1 should be 100% since layer 1 has a CQT of only 70after needling pass 1 and a CQT of 270 is desired. The desiredquantities may be arranged in a matrix such as that presented in FIG.14A. The layer numbers from FIG. 13 appear in the first column of FIG.14A. The second column, N, indicates a given layer in the set of layersas used in Equations 1 through 6. Note that the layer numbers of thefirst column do not match N because N is always 1 for the top layer inthe set of layers. The desired quantity of transport from each layerappears in the next column. Finally, an estimated layer thickness forall but the lowest layer in the set of layers appears in the lastcolumn. According to a preferred embodiment, the average layer thicknessfrom the previous needling step is used as the estimated layerthickness. Thus, 3.24 mm (t_(ave) from needling pass 1) appears in thedepth column for layers 2 (N=2) and 3 (N=1). A desired transport depthrelative to the lowest layer in the set of layers is then determined andplaced in the depth column of FIG. 14A for the lowest layer. In thisexample, this transport depth is D3₁ ² since N=3 for the lowest layer.According to a preferred embodiment, a table such as Table 1 is used todetermine the desired transport depth. Using this table, D3₁ ² must beat least 9.12 mm in order to achieve 100% permanent transport from layernumber 1 (N=3). The transport depth D1₁ ² relative to the top layer(N=1) is determined by summing the numbers that appear in the "Depth"column of FIG. 14A resulting in a desired value of 15.60 mm. The desiredtransport depth D1₁ for each needling pass appears in the "Desired D1₁ "column of FIG. 13. The desired transport depth D3₁ for each needlingpass appears in the "Desired D3₁ " column of FIG. 13.

The estimated transport depth D1₁ ² was then used in Equation 8 todetermine a bedplate setting for needling pass 2. According to apreferred embodiment, W^(i) of Equation 8 is determined by summingt_(ave) ^(i-1) and F^(i-1) as previously discussed in relation to FIG.11 and Equation 8. For needling pass 2, W² is 3.24 mm, and δ² is -1.2 mm(11.0 mm-6.36 mm+6.48 mm+3.24 mm-15.60 mm). The support position δ² isnegative which indicates a movement toward the felting needles relativeto the initial support position at needling pass 1. The support wasadjusted to δ² and the fibrous structure was subjected to needling pass2.

After the needling pass, appropriate measurements were made and Equation7 was used to calculate the actual transport depth D1₁ ² for the secondneedling pass, which was 14.61 mm. The actual transport depth D1₁ ^(i)for each needling pass appears in the "Actual D1₁ " column of FIG. 13.The actual quantities of permanent transport were then calculated andinserted in FIG. 13 according to previously described techniques. Forneedling pass 2, the desired transport depth was sufficient to achievethe desired quantities of 100% permanent fiber transport for all threelayers. The actual transport depth D3₁ ^(i) for each needling passcalculated according to Equation 6 appears in the "Actual D3₁ " columnof FIG. 13. The value for D3₁ ² of 9.37 mm verifies that 100% of thefiber transported from layer 1 (N=3) for needling pass 2 was permanentlytransported since this value is greater than 9.12 mm (see Table 1).

This process is repeated for each needling pass, examples of whichappear in FIGS. 14B and 14C wherein desired transport depths D1₁ forneedling passes 3 and 4 are determined. There are three layers in theset of layers, and the desired quantity of transport for the lowestlayer (N=3) in the set of layers is 70% for the remainder of theneedling passes since a uniform CQT of 270 is desired. The desiredquantity of 70% for the lowest layer (N=3) was achieved by ensuring thatD3₁ for each needling pass was between 7.00 and 8.06 mm, as required byTable 1. This was achieved by choosing a desired D3₁ of about 7.5 mm ateach needling pass. However, this number may be shifted toward the lowerlimit or the upper limit depending on the transport depth trend evidentfrom FIG. 13. For example, if it appears that the actual D3₁ isapproaching 8.06 mm for a given needling pass, the desired D3₁ for thenext needling pass may be shifted toward 7.00 mm (desired D3₁ <7.5 mm).Likewise, if D3₁ is approaching 7.00 mm for a given needling pass, thedesired value for the next pass may be shifted toward 8.06 mm (desiredD3₁ >7.5 mm). In such manner, transport depth may be adjusted to remainwithin the necessary limits to achieve the desired quantity of permanentfiber transport at each needling pass. The quantities of permanent fibertransport at each needling pass may thus be manipulated to achieve adesired CQT for each layer.

Still referring to FIG. 13, note that the actual CQT of 170 for layer 1is significantly lower than the desired CQT of 270. Achieving thedesired CQT for the first layer may be difficult, but is of littleconsequence since the first layer is usually removed during subsequentprocessing of the fibrous preform structure. The actual CQT closelytracks the desired CQT for layers 2 through 11. The actual CQT of layers12 and 13 is low, but may be increased to the desired CQT duringaddition of subsequent layers, or by subjecting the fibrous structure towalkout needling passes without adding layers.

Variations are possible without departing from the invention. Forexample, the desired CQT of FIG. 13 was the same for each layer.However, the desired CQT distribution may change from one layer to thenext. A fibrous preform structure having any such distribution may beachieved using the principles provided by this disclosure. Further, theprojected layer thicknesses for determining W^(i) and the fibertransport depth D1₁ ^(i) for each needling pass were based onmeasurements from the previous needling pass. The projected layerthicknesses may be determined using other techniques, such as derivingthem for a given needling pass from curves such as those presented inFIGS. 9 and 10. Similarly, W^(i) may be determined for a given needlingpass by measuring its value during formation of a substantially similarfibrous preform structure in a substantially similar process, oraccording to any other previously described or equivalent technique.

It is important to note that the effects of needling any given layeractually extend into several lower layers. In FIGS. 11 and 13, thelayers through which the Z-fiber bundle actually passes at each needlingstep are indicated by dashed lines. Fractional entries indicate thatfiber was transported through part of a layer during that needling step.Thus, needling of a subsequent layer increases the number of Z-fiberbundles in a lower layer. Increasing CQT for a subsequent layerincreases the amount of Z-fibers in a lower previously needled layer.

FIGS. 6, 8, 9, 10, and 11 are based on actual thickness measurements offibrous layers and fibrous preform structures. A small amount ofdeviation from one measurement to the next may be evident in thosefigures and is unavoidable. Measuring fibrous layer thickness andfibrous structure thickness accurately and repeatably is important inthe practice of the invention. Measuring thickness according to ASTM D1777-64 (reapproved 1975), "Standard Method for Measuring TextileMaterials", is preferred.

According to a further aspect of the invention, a Z-fiber bundledistribution may also be quantified, and a desired Z-fiber bundledistribution may be achieved. Referring again to FIG. 11, the number ofZ-fiber bundles penetrating each fibrous layer may be estimated bytotaling the number of needling passes that generate Z-fiber bundlespenetrating a given layer. If the number of Z-fiber bundles generatedduring a single needling pass is constant throughout formation of thefibrous structure, the number of Z-fiber bundles penetrating eachfibrous layer may be estimated by totaling the number of entries in agiven column, including any fractional entries (due to Z-fiber bundlespenetrating only a fraction of a fibrous layer). In FIG. 11, the totalsfor each fibrous layer are provided in the row entitled "Zn". The numberof Z-fiber bundles per layer is then estimated by multiplying Zn by thenumber of Z-fiber bundles per unit area generated by a single needlingpass. For example, assume that one hundred (100) Z-fiber bundles persquare centimeter were generated by each needling pass in the FIG. 11process. The number of Z-fiber bundles per layer are provided in the rowentitled ZpL which, in this example, is 100 times Zn for each layer. Thenumber of Z-fiber bundles may also include any Z-fiber bundles alreadycontained within the individual fibrous layers generated duringformation of those fibrous layers. For example, fibrous layers 2-31could already have fifty (50) Z-fiber bundles per square centimeter,which increases the total ZpL by that amount for those layers. Aspreviously discussed, the fibrous layers 2-31 each comprise threeunidirectional sub-layers of OPF fiber which are lightly needledtogether into a coherent layer. Layers 1 and 32 are an airlaid web. Bothtypes of layers already contain a number of Z-fiber bundles.

Referring now to FIG. 15, a cross-sectional view of a fibrous structure230 is presented according to an aspect of the invention. Fibrousstructure 230 comprises a multitude of superposed fibrous layers201-224. The multitude of fibrous layers 201-224 includes a first orupper group 232 of fibrous layers that has a lower fibrous layer 213 andan upper fibrous layer 224. According to a preferred embodiment, thefirst group 232 comprises a multitude of fibrous layers. Fibrousstructure 230 also comprises a multitude of Z-fiber bundles 240 thatpass between fibrous layers within the multitude of fibrous layers201-224. The Z-fiber bundles 240 are preferably generated by a needlingprocess as superposed fibrous layers are passed beneath a multitude offelting needles, the felting needles being cyclicly driven into thefibrous layers. As used herein, the term "needling density" for a givenlayer is the number of Z-fiber bundles originating in that layer perunit area. For example, Z-fiber bundles 319 originate in layer 219, andpenetrate subjacent layers 216-218. For the sake of clarity, only theZ-fiber bundles generated by a single felting needle are shown in FIG.15. Any of the previously described needling processes are suitable inthe practice of the invention. Each needling pass preferably generates aconstant number of Z-fiber bundles per unit area throughout formation ofthe fibrous structure, the unit area being normal to the Z-fiber bundleswithin the unit area. This may be accomplished by advancing the fibrousstructure beneath the felting needles at a constant speed, and bycyclicly driving the felting needles with a constant speed (punches persecond) into the fibrous structure. However, it is important to notethat the number of Z-fiber bundles generated per unit area during agiven needling pass may be varied throughout formation of the fibrousstructure. This may be accomplished by varying the advance speed of thefibrous structure, and/or the cyclic speed of the felting needles. Anysuch variations are considered to fall within the purview of theinvention.

A typical Z-fiber bundle generated by a needling process is presented inFIG. 16. Z-fiber bundle 319 originates in fibrous layer 219 andpenetrates subjacent fibrous layers 216-218. A barbed felting needle(not shown) penetrates fibrous layer 219 first and displaces fibers 252before withdrawing, thereby generating the Z-fiber bundle 319. Thedisplaced fibers may originate in any of the fibrous layers 216-219. Inthe example presented, fibers are pulled predominantly from layers 218and 219. The Z-fiber bundle 319 is only intended to be representative,and other Z-fiber bundles in fibrous layer 230 may penetrate a differentnumber of layers, and displace fibers similar to fibers 252 from more orfewer than two layers, depending on a variety of factors, including thetype of fiber and the needling process parameters.

Referring again to FIG. 15, each fibrous layer within the first group232 has portions 313-324 of the multitude of Z-fiber bundles 240originating in that fibrous layer and penetrating subjacent fibrouslayers without passing through all of the subjacent fibrous layers.Referring to Layer 219, for example, Z-fiber bundles 319 originate inlayer 219 and penetrate subjacent fibrous layers 216, 217 and 218without penetrating subjacent fibrous layers 201-215. A review of all ofthe fibrous layers 201-224 shows that each fibrous layer has portions ofZ-fiber bundles 301-324 originating in that layer and penetratingsubjacent fibrous layers. The portions of Z-fiber bundles 313-324 withinthe first group 232 penetrate an increasing number of fibrous layersfrom the lower fibrous layer 213 to the upper fibrous layer 224.According to a preferred embodiment, this trend causes the number ofZ-fibers per layer to increase within the first group 232 relative tothe lower fibrous layer 213. Comparing the sections of fibrous layers213 and 221 presented in FIG. 15, for example, reveals twenty-two (22)Z-fiber bundles in layer 213 as compared to thirty-six (36) in layer220. Increasing the number of Z-fiber bundles per layer, as shown,causes Z-fiber bundles from upper layers to reach down and increase thenumber of Z-fiber bundles in lower previously needled layers. Therefore,according to an aspect of the invention, each fibrous layer 213-224within the first group 232 has a number of Z-fiber bundles penetratingthat fibrous layer, and at least one fibrous layer 216-220 is disposedbetween the lower fibrous layer 213 and the upper fibrous layer 224 andhas a greater total quantity of Z-fiber bundles than the lower fibrouslayer 213 due at least in part to the number of fibrous layersincreasing from the lower fibrous layer 213 to the upper fibrous layer224.

Still referring to FIG. 15, the multitude of superposed fibrous layers201-224 may further include a second or lower group 234 of fibrouslayers that has a second lower fibrous layer 207 and a second upperfibrous layer 212. According to a preferred embodiment, the second group234 is disposed beneath the first group 232. Each fibrous layer withinthe second group 234 has portions 307-312 of the multitude of Z-fiberbundles 240 originating in that fibrous layer and penetrating subjacentfibrous layers without penetrating all of the subjacent fibrous layers201-206. The portions of Z-fiber bundles penetrate a decreasing numberof fibrous layers from the second lower fibrous layer 207 to the secondupper fibrous layer 212. Each fibrous layer 207-212 within the secondgroup 234 has a number of Z-fiber bundles penetrating that fibrouslayer, and at least one fibrous layer disposed between fibrous layer 207and the fibrous layer 212 has a lesser number of Z-fiber bundles thanfibrous layer 207 due at least in part to the number of fibrous layersdecreasing from the fibrous layer 207 to the second upper fibrous layer212. For example, fibrous layer 211 has twenty-one (21) Z-fiber bundlesand fibrous layer 207 has twenty-nine (29) Z-fiber bundles in thesections of those layers presented in FIG. 15. The number of Z-fiberbundles decreases because the number of fibrous layers penetrated byZ-fiber bundles decreases during subsequent needling passes, and Z-fiberbundles are not transported into lower previously needled layers thatwould have been transported had the number of layers remained constantor increased.

According to a particularly preferred embodiment, the second group 234is subjacent the first group 232 and the multitude of fibrous layers201-224 includes two outer fibrous layers 201 and 224 with severalintermediate layers 202-223 disposed between the two outer fibrouslayers 201 and 224. Each fibrous layer within the multitude of fibrouslayers 201-224 has a number of Z-fiber bundles penetrating that fibrouslayer. The number of Z-fiber bundles has a distribution that is greatestwithin the outer fibrous layers 201 and 224, and least at a point withinthe intermediate fibrous layers 202-223. The layer-by-layer distributionof the number of Z-fiber bundles per layer may be symmetric about thecenterline of the fibrous structure 230, as shown in FIG. 15. A bottomgroup of fibrous layers 236 has portions 301-306 of the multitude ofZ-fiber bundles 240 originating within fibrous layers 201-206. Note thatthe Z-fiber bundles 301-306 originate in one of layers 201-206, andpenetrate all of the subjacent fibrous layers. The number of fibrouslayers within group 236 depends on the desired Z-fiber distribution.

Referring now to FIG. 17, a radial section of a brake disk is presentedhaving an annular fibrous structure with two opposing generally planarfaces 244 and 246 bounded by an inside circumferential surface 250 andan outside circumferential surface 248. The fibrous structure has aZ-fiber distribution similar to FIG. 15, thus the two outer portions 242have more Z-fiber bundles than the inner portion 243. The fibrous layersare generally parallel to the outer generally planar faces 244 and 246.A binding matrix permeates the annular fibrous structure. Two outerportions 242 are adjacent planar faces 244 and 246, and an inner portion243 is disposed between the two outer portions 242. With some fibrouslayers, pulling fiber from a layer to create Z-fiber bundles tends todecrease the tensile strength of that layer in the brake disk, normal tothe direction of needling. Fibrous layers comprised of generallycontinuous filaments tend to exhibit these characteristics sinceneedling breaks the continuous fibers into shorter fibers. Tensilestrength of such a fibrous layer decreases as the amount of fiber pulledfrom that layer during needling increases. In fibrous structure 230, theZ-fiber bundles in outer portions 242 have greater lengths than theZ-fiber bundles in inner portion 243, which means that more fiber ispulled from outer portions 242 than inner portion 243. Therefore, thetwo outer portions 242 have a lesser tensile strength, normal to thedirection of needling, than the inner portion 243. On the other hand,increasing the number of Z-fiber bundles in a layer tends to increasethe resistance of that layer to mechanical wear. Therefore, outerportions 242 have greater resistance to wear than the inner portion 243.Also, increasing the number of Z-fiber bundles passing between layersincreases interlaminar shear strength of the brake disk so that outerportions 242 may also have a greater interlaminar shear strength thaninner portion 243 (keeping in mind that shear strength may reach amaximum and then decrease as needling increases). Therefore, the throughthickness Z-fiber distribution of FIG. 15 provides a functionalgradient. The brake disk 238 has the greatest resistance to wear andshear strength in the outer portions 242 adjacent the planar wearsurfaces 244 and 246, and the greatest tensile strength in the innerportion 243. The tensile strength and interlaminar shear strengthcharacteristics versus needling are preferably determined empiricallyfor a particular fibrous structure, needling process, and densificationmethod. Finally, the Z-fiber distribution of FIG. 15 may be enhanced byvarying the number of Z-fiber bundles per unit area originating withineach fibrous layer (needling density), the unit area being normal to theZ-fiber bundles. For example, fibrous layers in groups 232 and 236 couldbe subjected to a greater needling density than fibrous layers in group234 which would further amplify the through-thickness Z-fiber gradientdistribution. Increasing needling density may be accomplished variousways, including increasing the number of needle punches per unit areaduring a needling pass, and/or by subjecting a fibrous layer to at leasttwo needling passes without adding additional fibrous material.

Referring now to FIG. 18, a fibrous structure 430 is presented,according to another aspect of the invention. Fibrous structure 430comprises a multitude of superposed fibrous layers 401-424, a firstgroup of fibrous layers 432, and a second group of fibrous layers 434.Fibrous structure 430 is similar to fibrous structure 230, except thatsecond group 434 (similar to group 234) is an upper group, and firstgroup 432 (similar to group 232) is a lower group disposed beneath thesecond or upper group 434. The first group 432 has a lower fibrous layer404 and an upper fibrous layer 412, and preferably comprises a multitudeof fibrous layers. Fibrous structure 430 also comprises a multitude ofZ-fiber bundles 440 that pass between fibrous layers within themultitude of fibrous layers 401-424. As previously described in relationto FIG. 15, the Z-fiber bundles 440 are preferably generated by aneedling process as superposed fibrous layers are passed beneath amultitude of felting needles, the felting needles being cyclicly driveninto the fibrous layers. For the sake of clarity, only the Z-fiberbundles generated by a single felting needle are shown in FIG. 18. Abottom group of fibrous layers 436 is provided, similar to group 236 ofFIG. 15.

Still referring to FIG. 18, disposing the first group 432 beneath thesecond group 434 results in a fibrous structure having a differentZ-fiber distribution from that depicted in FIG. 15. Each fibrous layerwithin the multitude of fibrous layers 401-424 has a number of Z-fiberbundles penetrating that fibrous layer. The number of Z-fiber bundleshas a distribution that is least within the outer fibrous layers 401 and424, and greatest at a point within the intermediate fibrous layers402-423. The layer-by-layer distribution of the number of Z-fiberbundles per layer may be symmetric about the centerline of the fibrousstructure 430, as shown in FIG. 19. A symmetric distribution, asdepicted in FIGS. 15 and 19, is not necessary in the practice of theinvention. In addition, a fibrous structure according to the inventionmay comprise any number of fibrous layers of constant or varyingthickness, and the number of layers penetrated by Z-fiber bundles foreach fibrous layer may vary from those presented in FIGS. 15 and 19resulting in various Z-fiber distributions apparent to those skilled inthe art with knowledge of this disclosure. Any such variations areconsidered to fall within the purview of the invention.

Referring now to FIG. 19, a homogenous fibrous structure 630 ispresented, according to an aspect of the invention. Fibrous structure630 comprises a multitude of superposed fibrous layers 601-620 includinga lower fibrous layer 601 and an upper fibrous layer 620. Fibrous layers601 and 620 may also be referred to as the outermost fibrous layers. Amultitude of Z-fiber bundles 640 passes between fibrous layers withinthe multitude of fibrous layers 601-620. The multitude of superposedfibrous layers 601-602 includes an upper group of fibrous layers 632 anda lower group of fibrous layers 634 subjacent the upper group of fibrouslayers 632. The multitude of Z-fiber bundles 640 cohere the multitude offibrous layers 601-620. Each fibrous layer 606-620 within the uppergroup of fibrous layers 632 has a portion 706-720 of the multitude ofZ-fiber bundles originating in that fibrous layer and passing through anumber of fibrous layers disposed beneath that fibrous layer withoutpassing through all of the fibrous layers disposed beneath that fibrouslayer. The number of fibrous layers penetrated by the Z-fiber bundles706-720 is constant in the upper portion 632. Each fibrous layer 601-605within the lower group 634 of fibrous layers has a portion of themultitude of Z-fiber bundles originating in that fibrous layer andpassing through all fibrous layers disposed beneath that fibrous layer.

According to one embodiment, each fibrous layer has a number of Z-fiberbundles penetrating that fibrous layer, and the number of Z-fiberbundles is constant. In fibrous structure 630, for example, each portionof Z-fiber bundles 706-720 originating in the upper group 632 penetratesfive adjacent fibrous layers, resulting in each fibrous layer 601-620having forty-three (43) Z-fiber bundles in the cross-section shown. Thisis preferably achieved by maintaining a constant needling density withthe number of fibrous layers penetrated by Z-fiber bundles in the uppergroup 632 being constant, and by providing the necessary number offibrous layers in the lower group 634 to achieve the desired number ofZ-fiber bundles in the lower group layers. Note that in FIG. 19 thenumber of fibrous layers in the lower group 634 corresponds to thenumber of fibrous layers penetrated by Z-fiber bundles in the uppergroup 632 (five). The fibrous structure 630 has uniform physicalproperties throughout the thickness from fibrous layer 601-620. A brakedisk manufactured by densifying the fibrous structure 630 has uniformwear, strength and heat transfer characteristics through the thicknessof the disk. These characteristics remain constant as the disk wears dueto the through-thickness homogeneity.

According to another embodiment, a fibrous structure is provided havingZ-fiber bundles that penetrate a constant number of fibrous layersthrough the fibrous structure, each layer having a number of Z-fiberbundles penetrating that fibrous layer, and the number of Z-fiberbundles is non-constant within the fibrous structure. For example, theoutermost fibrous layers could have twice as many Z-fiber bundles thanan intermediate layer disposed between the outermost fibrous layers. Thenumber of Z-fiber bundles may be varied by varying the needling densitywhile forming the fibrous structure. A Z-fiber distribution functionallysimilar to the distribution presented in FIG. 15 may be achieved bydecreasing the needling density to a minimum while needling fibrouslayers within the inner portion 243. With such a method, a fibrous layerdisposed between the outermost fibrous layers has the least number ofZ-fiber bundles originating in that fibrous layer. Needling the firstlayers begins with an initial needling density, decreases to a minimumor least needling density while needling intermediate fibrous layers,and increases to a final needling density while needling final fibrouslayers. The initial and final needling densities may be generallyequivalent corresponding to a maximum needling density. If needlingdensity varies, the number of Z-fiber bundles per unit area originatingwithin each fibrous layer is non-constant. Thus, a predetermineddistribution of Z-fiber bundles may be achieved with the Z-fiber bundlespenetrating a constant number of layers in the upper group 632.

According to another aspect of the invention, a process is provided forforming a fibrous structure having a predetermined distribution ofZ-fiber bundles by varying Z-fiber bundle length. Fibrous structure 230of FIG. 15 will be referred to in describing the process, but theprocess is equally applicable to other fibrous structures having otherZ-fiber distributions, and any such variation is considered to fallwithin the purview of the invention. The process according to theinvention comprises the steps of superposing and needling a multitude offibrous layers 201-224 together in a series of needling passes thatgenerate a multitude of Z-fiber bundles 240 within the multitude offibrous layers 201-224, each needling pass generating a portion of themultitude of Z-fiber bundles that pass through a number of adjacentfibrous layers, each portion 302-324 of the multitude of Z-fiber bundles240 penetrating only those adjacent fibrous layers that need moreZ-fiber bundles to attain a predetermined number of Z-fiber bundleswithin each fibrous layer in the multitude of fibrous layers 201-224.For example, assume that the predetermined Z-fiber distributionpresented on Table 2 is desired in a fibrous structure, wherein N is thenumber of Z-fiber bundles per unit area generated during a singleneedling pass.

                  TABLE 2                                                         ______________________________________                                                    Number of                                                         Layer       Z-Fiber Bundles                                                   ______________________________________                                        1-3         6*N                                                               4-6         5*N                                                               7-9         4*N                                                               10-15       3*N                                                               16-18       4*N                                                               19-21       5*N                                                               22-24       6*N                                                               ______________________________________                                    

A fibrous structure 230 having this Z-fiber distribution may be formedby needling the fibrous layers in a series of needling passes with eachneedling pass generating a group of Z-fiber bundles in each layer thatpenetrate the number of layers presented on Table 3. For the purposes ofthis example, assume that the process begins by needling fibrous layer201 to at least one lower fibrous layer 200, since at least two fibrouslayers are generally required to generate Z-fiber bundles, unlessfibrous layer 201 is relatively thick. In addition, fibrous layers 225and 226 are provided in order to generate Z-fiber bundles in fibrouslayers 322-324 at the end of the process.

                  TABLE 3                                                         ______________________________________                                                       Number of                                                      Needling Pass  Fibrous Layers                                                 ______________________________________                                        1              2                                                              2              3                                                              3              4                                                              4              5                                                              5-8            6                                                               9-10          5                                                              11-12          4                                                              13-18          3                                                              19-22          4                                                              23-25          5                                                              26             4                                                              27             5                                                              28             4                                                              29             3                                                              ______________________________________                                    

Fibrous layers 200 and 225-226 are removed during subsequent processingwhich leaves fibrous structure 230 having the desired Z-fiberdistribution. Removing outer fibrous layers is routine during subsequentCVI/CVD processing. Although it may be desirable to provide the samenumber of removable layers on both sides of the fibrous structure 230since the same amount of material is preferably removed from both sidesduring subsequent processing. An additional fibrous layer is addedimmediately prior to each needling pass from needling pass 202 to 224.Fibrous layer 225 is subjected to an initial needling pass (needlingpass 25) followed by one walkout needling pass wherein the fibrousstructure is lowered relative to the multitude of felting needles andagain subjected to a needling pass (needling passes 26), and fibrouslayer 226 is subjected to an initial needling pass (needling pass 27)followed by two walkout needling passes (needling passes 28 and 29).Needling passes 25 and 26 generate Z-fiber bundles 325, and needlingpasses 27-29 generate Z-fiber bundles 326. The number of additionalfibrous layers and walkout passes depends on the desired Z-fiberdistribution in the outer layers. Note that the Z-fiber distribution ofTable 2 is symmetric about the midpoint of the fibrous structure, butthe number of adjacent fibrous layers for each needling pass is far fromsymmetric and varies within the multitude of fibrous layers 201-224.Thus, according to a preferred embodiment, the step of superposing andneedling the multitude of fibrous layers 201-224 begins with the bottomfibrous layer 200 and proceeds to a top fibrous layer 224 or beyond ifadditional fibrous layers are provided, such as fibrous layers 225 and226. A constant number of Z-fiber bundles per unit area is preferablygenerated during each needling pass. However, the number of Z-fiberbundles generated per unit area may vary throughout formation of thefibrous structure.

The process according to the invention is quite flexible, and may beused to generate many Z-fiber distributions. A fibrous structure havingthe Z-fiber distribution of the fibrous structure 430 (FIG. 18)presented in Table 4 may be produced by generating Z-fiber bundlesduring each needling pass as presented in Table 5. For the purposes ofthis example, assume that the process begins by needling fibrous layer401 to at least one lower fibrous layer 400, since at least two fibrouslayers are generally required to generate Z-fiber bundles, unlessfibrous layer 401 is relatively thick. In addition, fibrous layer 425 isprovided in order to generate Z-fiber bundles 525 in fibrous layer 424during needling pass 25 at the end of the process. Fibrous layers 400and 425 are removed during subsequent processing which leaves fibrousstructure 430 having the desired Z-fiber distribution. As with fibrousstructure 230, it may be desirable to provide the same number ofremovable layers on both sides since the same amount of material ispreferably removed from both sides of the fibrous structure duringsubsequent processing. An additional fibrous layer is added immediatelyprior to each needling pass from needling pass 2 to 24.

                  TABLE 4                                                         ______________________________________                                                     Number of                                                        Layer        Z-Fiber Bundles                                                  ______________________________________                                        201-203      3*N                                                              204-206      4*N                                                              207-209      5*N                                                              210-215      6*N                                                              216-218      5*N                                                              219-221      4*N                                                              222-224      3*N                                                              ______________________________________                                    

                  TABLE 5                                                         ______________________________________                                                       Number of                                                      Needling Pass  Fibrous Layers                                                 ______________________________________                                        1              2                                                              2-6            3                                                               7-10          4                                                              11-13          5                                                              14-20          6                                                              21-22          5                                                              23-24          4                                                              25             2                                                              ______________________________________                                    

A fibrous structure having the Z-fiber distribution of the fibrousstructure 630 of FIG. 19 may be produced by generating Z-fiber bundlesduring each needling pass as presented in Table 6. Fibrous structure 630has a constant number of Z-fiber bundles in each fibrous layer 601-620,which may be quantified as 5*N. For the purposes of this example, assumethat the process begins by needling fibrous layer 601 to at least onelower fibrous layer 600, since at least two fibrous layers are generallyrequired to generate Z-fiber bundles, unless fibrous layer 601 isrelatively thick. In addition, fibrous layer 621 is provided in order togenerate Z-fiber bundles 721 in fibrous layer 620 during needling pass21 at the end of the process. Fibrous layers 600 and 621 are removedduring subsequent processing which leaves fibrous structure 630 havingthe desired Z-fiber distribution. An additional fibrous layer is addedimmediately prior to each needling pass from needling pass 2 to 20. TheTable 6 values generate a homogenous fibrous structure if needlingdensity is constant. As previously described in relation to FIG. 19, theneedling density may be non-constant and varied during the needlingprocess to generate a predetermined Z-fiber distribution.

                  TABLE 6                                                         ______________________________________                                                       Number of                                                      Needling Pass  Fibrous Layers                                                 ______________________________________                                        1              2                                                              2              2                                                              3              3                                                              4              4                                                              5-20           5                                                              21             2                                                              ______________________________________                                    

In the method according to the invention, the multitude of fibrouslayers are cohered in a series of needling passes, each needling passcomprising the steps of superposing at least one fibrous layer overfibrous layers previously cohered by previous needling passes. Z-fiberbundles are generated that extend from at least one fibrous layer intothe previously cohered fibrous layers, each of the previously coheredfibrous layers receiving Z-fiber bundles during subsequent needlingpasses until the number of Z-fiber bundles in that fibrous layer reach apredetermined number of Z-fiber bundles for that layer, each fibrouslayer in the multitude of fibrous layers thereby having a predeterminednumber of Z-fiber bundles for that fibrous layer. The Z-fiber bundlesgenerated by at least a portion of the needling passes do not penetrateall the way through the previously needled fibrous layers. As describedthus far, the number of adjacent fibrous layers penetrated by the groupsof Z-fiber bundles has been expressed as an integer in order tofacilitate describing the invention. The number of adjacent layers mayalso include fractions of fibrous layers as necessary to achieve adesired Z-fiber distribution. In practice, the number of adjacentfibrous layers penetrated by the groups of Z-fiber bundles willgenerally include fractions of fibrous layers.

The process described in relation to FIGS. 15-19 for forming a fibrousstructure with a desired Z-fiber distribution may be used with orwithout considering compaction and minimum transport, as described inrelation to FIGS. 1-14. Considering minimum fiber transport is notnecessary if transported fiber has little or no tendency to pull back tothe fibrous fiber layer/s from which it originates. Consideringcompaction of previously needled layers is not necessary if the fibrouslayers used in a particular process have little or no tendency tocompact during subsequent needling passes. If compaction is a factor,Equation 7 is preferably used to calculate actual transport depth duringa given needling pass, and Equation 8 is preferably used to calculatethe support setting for the next needling pass. The desired transportdepth, D1₁ ^(i), in Equation 8 is determined by adding together thelayer thicknesses of the number of layers (including a fractional layerif the number includes a fraction of a layer) to be penetrated byZ-fiber bundles in the next needling pass. The individual layerthicknesses may be estimated by any of the methods disclosed in relationto FIGS. 1-14. The process according to the invention may proceed in anactively controlled "predictor-corrector" technique, alternating betweenEquations 7 and 8, as previously described in relation to FIG. 11.

According to a preferred embodiment, all or some of the fibrous layersare comprised of three unidirectional sub-layers of continuous OPF fiberwhich are lightly needled together into a coherent layer, as previouslydescribed. The needling process utilizes a perforated support, as shownin FIGS. 4A and 4B, and compaction and minimum transport are preferablyconsidered. However, it is important to note that minimum transport,compaction characteristics, and fiber transport characteristics for agiven needling process are determined empirically for that process. Suchcharacteristics may be determined according to the principles disclosedherein.

Finally, still referring to the process described in relation to FIGS.15-19, determining a quantity of fiber transport from each layer, andthe set of layers that fiber is transported from, or forming a fibrousstructure having a desired CQT for each layer is not necessary. However,quantifying CQT is preferred since it provides an indication of wheretransported fiber originates, and how much fiber is transported fromeach layer in the fibrous structure. As previously described,transporting fiber from a fibrous layer affects the mechanical strengthof that fibrous layer. Maintaining CQT within a prescribed range in atleast a portion of the fibrous structure is preferred. A fibrousstructure for a brake disk according to FIG. 15 preferably has a ZpL inthe range of 650-750 Z-fiber bundles per square centimeter at the wearsurfaces, and a minimum ZpL in the range of about 500-600 close to thecenterline (including any Z-fiber bundles already present in eachfibrous layer generated during formation of that layer). The CQT foreach layer between the wear surfaces is preferably maintained in therange of 200-500, when calculated according to FIG. 11 (preferably usingEquation 6a). A fibrous structure for a brake disk according to FIG. 19may have a generally constant ZpL of about 625 Z-fiber bundles persquare centimeter and a CQT in the range 250-350.

The number of Z-fiber bundles per unit area normal to the direction ofneedling may be varied in a given layer by varying the needling density,by varying transport depth as disclosed herein, and by a combination ofvarying needling density and transport depth. The invention may be usedwith any of these approaches, and any such variations are considered tofall within the purview of the invention. Increasing needling density ina process according to the invention may be accomplished various ways,including increasing the number of needle punches per unit area during aneedling pass, and/or by subjecting a fibrous layer to at least twoneedling passes without adding addtional fibrous material.

A gradient Z-fiber distribution for the brake disk 238 of FIG. 17 may beachieved other ways. Such a process may comprise the steps of forming amultitude of Z-fiber bundles passing between fibrous layers within amultitude of superposed fibrous layers in a series of needling passesthereby forming a fibrous structure having two outer portions 242 and aninner portion 243 between the two outer portions 242, each needling passhaving a needling density, the needling density being greater whenforming the two outer portions 242 than when forming the inner portion Abrake disk may be formed from such a fibrous structure by forming abinding matrix permeating the fibrous layers. The two outer portions 242have more Z-fiber bundles than the inner portion 243. A brake disk madeaccordingly may have a lesser tensile strength parallel to the surfaces244 and 246 and greater resistance to wear in the outer portions 242than in the inner portion 243, as previously described in relation toFIG. 17. The needling density may be increased by subjecting at leastone fibrous layer in the outer portions 242 to at least two needlingpasses before adding additional fibrous material. The needling densitymay also be changed by changing the number of needle punches per unitarea, or by a combination of these two techniques. Any such process maybe used in conjunction with any of the processes discussed thus far.

It is evident that many variations are possible without departing fromthe true scope and spirit of the invention as defined by the claims thatfollow.

We claim:
 1. A process for forming a fibrous structure, comprising thesteps of:superposing and needling a multitude of fibrous layers togetherin a series of needling passes that generate a multitude of Z-fiberbundles within said multitude of fibrous layers, each needling passgenerating a portion of said multitude of Z-fiber bundles that penetrateadjacent fibrous layers, each portion of said multitude of Z-fiberbundles penetrating only those adjacent fibrous layers that need moreZ-fiber bundles to attain a predetermined number of Z-fiber bundleswithin each fibrous layer in said multitude of fibrous layers.
 2. Theprocess of claim 1, wherein said step of superposing and needling saidmultitude of fibrous layers begins with a bottom fibrous layer andproceeds to a top fibrous layer, a portion of said multitude of Z-fiberbundles originating in said top layer and penetrating at least onefibrous layer disposed beneath said top layer without penetrating all ofsaid fibrous layers disposed beneath said top layer.
 3. The process ofclaim 1, wherein said multitude of fibrous layers includes an uppergroup of fibrous layers and a lower group of fibrous layers disposedbeneath said upper group, a portion of said multitude of Z-fiber bundlesoriginating within fibrous layers in said upper group without extendingthrough all of said fibrous layers disposed beneath said upper group. 4.The process of claim 1, wherein said predetermined number of Z-fiberbundles for each fibrous layer is constant throughout said multitude offibrous layers.
 5. The process of claim 1, wherein said predeterminednumber of Z-fiber bundles for each fibrous layer varies within saidmultitude of fibrous layers.
 6. The process of claim 1, wherein fibrouslayers previously needled during said step of superposing and needlingsaid multitude of fibrous layers compact during needling of subsequentlyadded fibrous layers.
 7. The process of claim 1, wherein Z-fibersbundles generated during each needling pass have lengths that are thesum of estimated fibrous layer thicknesses of adjacent fibrous layerspenetrated by those Z-fiber bundles for that needling pass, including afractional fibrous layer thickness if said number of adjacent fibrouslayers includes a fraction.
 8. The process of claim 1, wherein aconstant number of Z-fiber bundles per unit area is generated duringeach needling pass, said unit area being normal to said Z-fiber bundleswithin said unit area.
 9. The process of claim 8, wherein each of saidportions of Z-fiber bundles penetrate the same number of adjacentfibrous layers.
 10. The process of claim 8, wherein at least one portionof said Z-fiber bundles penetrate a different number of adjacent fibrouslayers than at least one other portion of said Z-fiber bundles.
 11. Theprocess of claim 8, wherein said number of adjacent fibrous layers is aninteger for at least one needling pass.
 12. The process of claim 1,wherein said number of Z-fiber bundles per unit area generated during atleast one needling pass is different than said number of Z-fiber bundlesper unit area generated during at least one other needling pass, saidunit area being normal to said Z-fiber bundles within said unit area.13. A process for forming a fibrous structure, comprising the stepsof:cohering a multitude of fibrous layers in a series of needlingpasses, each needling pass comprising the steps of superposing at leastone fibrous layer over fibrous layers previously cohered by previousneedling passes, and generating Z-fiber bundles that extend from said atleast one fibrous layer into said previously cohered fibrous layers,each of said previously cohered fibrous layers receiving Z-fiber bundlesduring subsequent needling passes until the number of Z-fiber bundles inthat fibrous layer reach a predetermined number of Z-fiber bundles forthat layer, each fibrous layer in said multitude of fibrous layersthereby having a predetermined number of Z-fiber bundles for thatfibrous layer.
 14. The process of claim 13, wherein said Z-fiber bundlesgenerated by at least a portion of said needling passes do not penetrateall the way through said previously needled fibrous layers.
 15. Theprocess of claim 13, wherein said predetermined number of Z-fiberbundles for each fibrous layer is constant throughout said multitude offibrous layers.
 16. The process of claim 13, wherein said predeterminednumber of Z-fiber bundles for each fibrous layer varies within saidmultitude of fibrous layers.
 17. The process of claim 13, wherein saidpreviously needled fibrous layers compact during subsequent needlingpasses.
 18. The process of claim 13, wherein a constant number ofZ-fiber bundles per unit area is generated during each needling pass,said unit area being normal to said Z-fiber bundles within said unitarea.
 19. The process of claim 13, wherein said number of Z-fiberbundles per unit area generated during at least one needling pass isdifferent than said number of Z-fiber bundles per unit area generatedduring at least one other needling pass, said unit area being normal tosaid Z-fiber bundles within said unit area.
 20. A process for forming afibrous structure, comprising the steps of:determining a desired numberof adjacent fibrous layers to be penetrated by Z-fiber bundles for eachneedling pass such that the sum of Z-fiber bundles generated in eachlayer by each needling pass equals a predetermined number of Z-fiberbundles for that fibrous layer; and, superposing and needling saidmultitude of fibrous layers together in a series of needling passes thatgenerate said multitude of Z-fiber bundles within said multitude offibrous layers, said Z-fiber bundles generated by each needling passpenetrating said desired number of adjacent fibrous layers for thatneedling pass, thereby achieving said desired number of Z-fiber bundlesfor each fibrous layer in said multitude of fibrous layers.
 21. Theprocess of claim 20, wherein a constant number of Z-fiber bundles perunit area is generated during each needling pass, and fibrous layerspreviously needled during said step of superposing and needling saidmultitude of fibrous layers compact during needling of subsequentlyadded fibrous layers, said unit area being normal to said Z-fiberbundles within said unit area.
 22. A process for making a fibrousstructure, comprising the steps of:forming a multitude of Z-fiberbundles passing between fibrous layers within a multitude of superposedfibrous layers, said multitude of superposed fibrous layers including afirst group having a lower fibrous layer and an upper fibrous layer,each fibrous layer within said first group having a portion of saidmultitude of Z-fiber bundles originating in that fibrous layer andpassing through a number of fibrous layers disposed beneath that fibrouslayer without passing through all of said fibrous layers disposedbeneath that fibrous layer, said number of fibrous layers increasingfrom said lower fibrous layer to said upper fibrous layer.
 23. Theprocess of claim 22, wherein said first group comprises a multitude offibrous layers.
 24. The process of claim 22, wherein each fibrous layerwithin said first group has a number of Z-fiber bundles penetrating thatfibrous layer, due at least in part to said number of fibrous layersincreasing from said lower fibrous layer to said upper fibrous layer.25. The process of claim 22, wherein the number of Z-fiber bundles perunit area originating within each fibrous layer is constant within saidfirst group, said unit area being normal to said Z-fiber bundles withinsaid unit area.
 26. The process of claim 25, wherein the number ofZ-fiber bundles per unit area originating within each fibrous layervaries within said first group, said unit area being normal to saidZ-fiber bundles within said unit area.
 27. The process of claim 22,wherein said multitude of superposed fibrous layers includes a secondgroup having a second lower fibrous layer and a second upper fibrouslayer,each fibrous layer within said second group having a portion ofsaid multitude of Z-fiber bundles originating in that fibrous layer andpassing through a number of fibrous layers disposed beneath that fibrouslayer without passing through all of said fibrous layers disposedbeneath that fibrous layer, said number of fibrous layers decreasingfrom said second lower fibrous layer to said second upper fibrous layer.28. The process of claim 27, wherein each fibrous layer within saidsecond group has a number of Z-fiber bundles penetrating that fibrouslayer, due at least in part to said number of fibrous layers increasingfrom said second lower fibrous layer to said second upper fibrous layer.29. The process of claim 27, wherein said second group is disposedbeneath said first group.
 30. The process of claim 27, wherein saidfirst group is disposed beneath said second group.
 31. The process ofclaim 27, wherein said second group is subjacent said first group;andeach fibrous layer within said multitude of fibrous layers has anumber of Z-fiber bundles penetrating that fibrous layer, said number ofZ-fiber bundles being greatest within said outer fibrous layers andleast within said intermediate fibrous layers.